remove the Opm::FluidSystems namespace

this has mildly annoyed me for quite some time, and finally managed to
bring myself to changing it: The Opm::FluidSystems namespace is pretty
useless because the number of classes contained within it is quite
small and mismatch between the naming convention of the file names the
actual classes is somewhat confusing IMO. Thus, this patch changes the
naming of fluid systems from `Opm::FluidSystems::Foo` to
`Opm::FooFluidSystem`.

(also, flat hierarchies currently seem to be popular with the cool
people!?)

this patch requires some simple mop-ups for `ewoms` and `opm-simulators`.

opm/material/fluidsystems/SinglePhaseFluidSystem.hpp

@@ -22,7 +22,7 @@
 */
 /*!
  * \file
- * \copydoc Opm::FluidSystems::SinglePhase
+ * \copydoc Opm::SinglePhaseFluidSystem
  */
 #ifndef OPM_SINGLE_PHASE_FLUIDSYSTEM_HPP
 #define OPM_SINGLE_PHASE_FLUIDSYSTEM_HPP
@@ -43,7 +43,6 @@
 #include <cassert>
 
 namespace Opm {
-namespace FluidSystems {
 
 /*!
  * \ingroup Fluidsystems
@@ -55,10 +54,10 @@ namespace FluidSystems {
  * Opm::GasPhase<Component> may be used.
  */
 template <class Scalar, class Fluid>
-class SinglePhase
-    : public BaseFluidSystem<Scalar, SinglePhase<Scalar, Fluid> >
+class SinglePhaseFluidSystem
+    : public BaseFluidSystem<Scalar, SinglePhaseFluidSystem<Scalar, Fluid> >
 {
-    typedef SinglePhase<Scalar, Fluid> ThisType;
+    typedef SinglePhaseFluidSystem<Scalar, Fluid> ThisType;
     typedef BaseFluidSystem<Scalar, ThisType> Base;
 
 public:
@@ -269,6 +268,6 @@ public:
     }
 };
 
-}} // namespace Opm, FluidSystems
+} // namespace Opm
 
 #endif