Merge branch 'feature/simplify-porousmedium-fluxvars' into 'next'

dumux/discretization/fluxvariablesbase.hh

dumux/porousmediumflow/1p/implicit/properties.hh

dumux/porousmediumflow/1p/implicit/propertydefaults.hh

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

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

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

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

dumux/porousmediumflow/3p/implicit/properties.hh

dumux/porousmediumflow/3p/implicit/propertydefaults.hh

dumux/porousmediumflow/implicit/fluxvariables.hh

dumux/porousmediumflow/richards/implicit/properties.hh

dumux/porousmediumflow/richards/implicit/propertydefaults.hh