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/co2injectionflash.hh

@@ -50,11 +50,11 @@ public:
      * \brief Guess initial values for all quantities.
      */
     template <class FluidState, class ComponentVector>
-    static void guessInitial(FluidState &fluidState, const ComponentVector &globalMolarities)
+    static void guessInitial(FluidState& fluidState, const ComponentVector& globalMolarities)
     {
         ParentType::guessInitial(fluidState, globalMolarities);
 
-        for (int phaseIdx = 0; phaseIdx < numPhases; ++phaseIdx) {
+        for (unsigned phaseIdx = 0; phaseIdx < numPhases; ++phaseIdx) {
             // pressure. use something close to the reservoir pressure as initial guess
             fluidState.setPressure(phaseIdx, 100e5);
         }