flash model: enable volume variable caching and thermodynamic hints in the generic code

These used to be needed to be enabled by the problem, but doing it in
the generic code is more fool proof and thus better.

On my laptop, volume variable caching caused the linearization time
for first time step of the isothermal vertex-centered CO2 injection
problem using the flash model go from 33.7 to 15.7 seconds, and
enabling thermodynamic hints in addition yielded a linearization time
of "only" 9.5 seconds. (that is for a debug build compiled with clang
3.3.)

examples/tutorial1problem.hh

@@ -303,3 +303,4 @@ private:
 } // namespace Ewoms
 
 #endif
+