adapt construction of to EclipseWriter

i.e., don't pass around the UnstructuredGrid object directly, but use
the "generic" interface which is also used in the case Dune::CpGrid.
pass the EclipseState and the PhaseUsage objects to EclipseWriter
Merge pull request #167 from andlaus/do_not_pass_shared_ptr_to_EclipseWriter

pass a reference to the UnstructuredGrid to EclipseWriter instead of a shared_ptr