[2p2c] Use switchable primary variables

dumux/porousmediumflow/2p2c/implicit/model.hh

...x/porousmediumflow/2p2c/implicit/primaryvariableswitch.hh

dumux/porousmediumflow/2p2c/implicit/propertydefaults.hh

dumux/porousmediumflow/2p2c/implicit/volumevariables.hh

test/porousmediumflow/2p2c/implicit/injectionproblem.hh

.../porousmediumflow/2p2c/implicit/injectionspatialparams.hh