fix most pedantic compiler warnings in the basic infrastructure

i.e., using clang 3.8 to compile the test suite with the following
flags:

```
-Weverything
-Wno-documentation
-Wno-documentation-unknown-command
-Wno-c++98-compat
-Wno-c++98-compat-pedantic
-Wno-undef
-Wno-padded
-Wno-global-constructors
-Wno-exit-time-destructors
-Wno-weak-vtables
-Wno-float-equal
```

should not produce any warnings anymore. In my opinion the only flag
which would produce beneficial warnings is -Wdocumentation. This has
not been fixed in this patch because writing documentation is left for
another day (or, more likely, year).

note that this patch consists of a heavy dose of the OPM_UNUSED macro
and plenty of static_casts (to fix signedness issues). Fixing the
singedness issues were quite a nightmare and the fact that the Dune
API is quite inconsistent in that regard was not exactly helpful. :/

Finally this patch includes quite a few formatting changes (e.g., all
occurences of 'T &t' should be changed to `T& t`) and some fixes for
minor issues which I've found during the excercise.

I've made sure that all unit tests the test suite still pass
successfully and I've made sure that flow_ebos still works for Norne
and that it did not regress w.r.t. performance.

(Note that this patch does not fix compiler warnings triggered `ebos`
and `flow_ebos` but only those caused by the basic infrastructure or
the unit tests.)

v2: fix the warnings that occur if the dune-localfunctions module is
    not available. thanks to [at]atgeirr for testing.
v3: fix dune 2.3 build issue

examples/problems/diffusionproblem.hh

@@ -30,7 +30,6 @@
 
 #include <ewoms/models/ncp/ncpproperties.hh>
 
-#include <dune/grid/yaspgrid.hh>
 #include <ewoms/io/cubegridmanager.hh>
 
 #include <opm/material/fluidmatrixinteractions/LinearMaterial.hpp>
@@ -38,7 +37,9 @@
 #include <opm/material/fluidsystems/H2ON2FluidSystem.hpp>
 #include <opm/material/fluidstates/CompositionalFluidState.hpp>
 #include <opm/material/constraintsolvers/ComputeFromReferencePhase.hpp>
+#include <opm/material/common/Unused.hpp>
 
+#include <dune/grid/yaspgrid.hh>
 #include <dune/common/version.hh>
 #include <dune/common/fvector.hh>
 #include <dune/common/fmatrix.hh>
@@ -174,7 +175,7 @@ public:
     /*!
      * \copydoc Doxygen::defaultProblemConstructor
      */
-    DiffusionProblem(Simulator &simulator)
+    DiffusionProblem(Simulator& simulator)
         : ParentType(simulator)
     { }
 
@@ -237,30 +238,37 @@ public:
      * \copydoc FvBaseMultiPhaseProblem::intrinsicPermeability
      */
     template <class Context>
-    const DimMatrix &intrinsicPermeability(const Context &context, unsigned spaceIdx,
-                                           unsigned timeIdx) const
+    const DimMatrix& intrinsicPermeability(const Context& OPM_UNUSED context,
+                                           unsigned OPM_UNUSED spaceIdx,
+                                           unsigned OPM_UNUSED timeIdx) const
     { return K_; }
 
     /*!
      * \copydoc FvBaseMultiPhaseProblem::porosity
      */
     template <class Context>
-    Scalar porosity(const Context &context, unsigned spaceIdx, unsigned timeIdx) const
+    Scalar porosity(const Context& OPM_UNUSED context,
+                    unsigned OPM_UNUSED spaceIdx,
+                    unsigned OPM_UNUSED timeIdx) const
     { return 0.35; }
 
     /*!
      * \copydoc FvBaseMultiPhaseProblem::materialLawParams
      */
     template <class Context>
-    const MaterialLawParams &materialLawParams(const Context &context,
-                                               unsigned spaceIdx, unsigned timeIdx) const
+    const MaterialLawParams&
+    materialLawParams(const Context& OPM_UNUSED context,
+                      unsigned OPM_UNUSED spaceIdx,
+                      unsigned OPM_UNUSED timeIdx) const
     { return materialParams_; }
 
     /*!
      * \copydoc FvBaseMultiPhaseProblem::temperature
      */
     template <class Context>
-    Scalar temperature(const Context &context, unsigned spaceIdx, unsigned timeIdx) const
+    Scalar temperature(const Context& OPM_UNUSED context,
+                       unsigned OPM_UNUSED spaceIdx,
+                       unsigned OPM_UNUSED timeIdx) const
     { return temperature_; }
 
     //! \}
@@ -276,8 +284,10 @@ public:
      * This problem sets no-flow boundaries everywhere.
      */
     template <class Context>
-    void boundary(BoundaryRateVector &values, const Context &context,
-                  unsigned spaceIdx, unsigned timeIdx) const
+    void boundary(BoundaryRateVector& values,
+                  const Context& OPM_UNUSED context,
+                  unsigned OPM_UNUSED spaceIdx,
+                  unsigned OPM_UNUSED timeIdx) const
     { values.setNoFlow(); }
 
     //! \}
@@ -291,10 +301,12 @@ public:
      * \copydoc FvBaseProblem::initial
      */
     template <class Context>
-    void initial(PrimaryVariables &values, const Context &context, unsigned spaceIdx,
+    void initial(PrimaryVariables& values,
+                 const Context& context,
+                 unsigned spaceIdx,
                  unsigned timeIdx) const
     {
-        const auto &pos = context.pos(spaceIdx, timeIdx);
+        const auto& pos = context.pos(spaceIdx, timeIdx);
         if (onLeftSide_(pos))
             values.assignNaive(leftInitialFluidState_);
         else
@@ -308,14 +320,16 @@ public:
      * everywhere.
      */
     template <class Context>
-    void source(RateVector &rate, const Context &context, unsigned spaceIdx,
-                unsigned timeIdx) const
+    void source(RateVector& rate,
+                const Context& OPM_UNUSED context,
+                unsigned OPM_UNUSED spaceIdx,
+                unsigned OPM_UNUSED timeIdx) const
     { rate = Scalar(0.0); }
 
     //! \}
 
 private:
-    bool onLeftSide_(const GlobalPosition &pos) const
+    bool onLeftSide_(const GlobalPosition& pos) const
     { return pos[0] < (this->boundingBoxMin()[0] + this->boundingBoxMax()[0]) / 2; }
 
     void setupInitialFluidStates_()