[fluxcache] Simplify and at the same time implement all combinations of caches

dumux/discretization/cellcentered/mpfa/darcyslaw.hh

dumux/discretization/cellcentered/mpfa/fickslaw.hh

...cretization/cellcentered/mpfa/fluxvariablescachefiller.hh

dumux/discretization/cellcentered/mpfa/fourierslaw.hh

dumux/discretization/cellcentered/tpfa/darcyslaw.hh

dumux/discretization/cellcentered/tpfa/fickslaw.hh

...cretization/cellcentered/tpfa/fluxvariablescachefiller.hh

dumux/discretization/cellcentered/tpfa/fourierslaw.hh

dumux/discretization/fluxvariablescaching.hh

dumux/discretization/staggered/freeflow/fickslaw.hh

dumux/discretization/staggered/freeflow/fourierslaw.hh

dumux/discretization/stationaryvelocityfield.hh

dumux/porousmediumflow/implicit/fluxvariablescache.hh