[2p] make incompressible tests the new isothermal 2p tests

dumux/porousmediumflow/2p/implicit/properties.hh

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

...orousmediumflow/2p/implicit/incompressible/CMakeLists.txt

...ousmediumflow/2p/implicit/incompressible/spatialparams.hh

...porousmediumflow/2p/implicit/incompressible/test_2p_cc.cc

test/porousmediumflow/2p/implicit/lensproblem.hh

test/porousmediumflow/2p/implicit/lensspatialparams.hh

test/porousmediumflow/2p/implicit/test_box2p.cc

test/porousmediumflow/2p/implicit/test_box2p.input

test/porousmediumflow/2p/implicit/test_cc2p.cc

test/porousmediumflow/2p/implicit/test_cc2p.input