Merge branch 'cleanup/staggered-ff' into 'next'

dumux/assembly/staggeredlocalassembler.hh

dumux/discretization/staggered/freeflow/boundarytypes.hh

dumux/discretization/staggered/freeflow/properties.hh

dumux/discretization/staggered/freeflow/velocityoutput.hh

dumux/discretization/staggered/properties.hh

dumux/freeflow/CMakeLists.txt

dumux/freeflow/navierstokes/CMakeLists.txt

dumux/freeflow/navierstokes/fluxvariables.hh

dumux/freeflow/navierstokes/fluxvariablescache.hh

dumux/freeflow/navierstokes/indices.hh

dumux/freeflow/navierstokes/localresidual.hh

dumux/freeflow/navierstokes/model.hh

dumux/freeflow/navierstokes/problem.hh

dumux/freeflow/navierstokes/staggered/fluxvariables.hh

dumux/freeflow/navierstokes/staggered/localresidual.hh

dumux/freeflow/navierstokes/volumevariables.hh

dumux/freeflow/navierstokes/vtkoutputfields.hh

dumux/freeflow/navierstokesnc/CMakeLists.txt

dumux/freeflow/navierstokesnc/fluxvariables.hh

dumux/freeflow/navierstokesnc/indices.hh