[2p1c] harmonize sources, input and executables

test/porousmediumflow/2p1c/implicit/CMakeLists.txt

test/porousmediumflow/2p1c/implicit/main.cc

test/porousmediumflow/2p1c/implicit/params_box.input

test/porousmediumflow/2p1c/implicit/params_tpfa.input

test/porousmediumflow/2p1c/implicit/problem.hh

test/porousmediumflow/2p1c/implicit/spatialparams.hh

...ences/test_2p1cni_steaminjection_gaswet_box-reference.vtu

...nces/test_2p1cni_steaminjection_gaswet_tpfa-reference.vtu

...ces/test_2p1cni_steaminjection_waterwet_box-reference.vtu

...es/test_2p1cni_steaminjection_waterwet_tpfa-reference.vtu