fully implicit simulators: various cleanups

these are mostly stylistic: the function bodies of most new methods
have been moved to the _impl.hpp file and the Simulator classes are
now templated on the grid type, so it should be not too hard to switch
them to Dune::CpGrid.

examples/sim_poly_fi2p_comp_ad.cpp

@@ -224,7 +224,7 @@ try
     SimulatorReport fullReport;
     // Create and run simulator.
     Opm::DerivedGeology geology(*grid->c_grid(), *new_props, eclipseState, grav);
-    SimulatorFullyImplicitCompressiblePolymer
+    SimulatorFullyImplicitCompressiblePolymer<UnstructuredGrid>
         simulator(param,
                   *grid->c_grid(),
                   geology,