Added first draft of monolithic test case

dune/phasefield/base_potentials.hh

dune/phasefield/chns_base.hh

dune/phasefield/ff_chns.hh

dune/phasefield/localoperator/fem_base_chns.hh

dune/phasefield/localoperator/fem_fs_chns_cahnhilliard.hh

dune/phasefield/localoperator/fem_fs_chns_monolithic.hh

dune/phasefield/localoperator/fem_fs_chns_navierstokes.hh

.../phasefield/localoperator/pf_diff_estimator_monolithic.hh

src/CMakeLists.txt

src/example_2p_fs/2p_driver.hh

src/example_2p_fs_monolithic/2p_driver.hh

src/example_2p_fs_monolithic/2p_fs_monolithic.cc

src/example_2p_fs_monolithic/CMakeLists.txt

src/example_2p_fs_monolithic/chns2p.ini

...mple_2p_fs_monolithic/grids/rectangle_periodic_coarse.msh