Simplified local operators, fixed doublewell_limited

dune/phasefield/base_parameters.hh

dune/phasefield/base_potentials.hh

dune/phasefield/boundary_fn.hh

dune/phasefield/chns_base.hh

dune/phasefield/fs_chns_r.hh

dune/phasefield/initial_fn.hh

dune/phasefield/localoperator/fem_base_chns.hh

dune/phasefield/localoperator/fem_fs_chns_r_cahnhilliard.hh

dune/phasefield/localoperator/fem_fs_chns_r_instatstokes.hh

src/test_2pchns_r/2p_driver.hh

src/test_2pchns_r/2pchns_r.ini