[folder structure] rename files in implicit/box, adapt includes

dumux/freeflow/stokes/stokesproperties.hh

dumux/geomechanics/el1p2c/el1p2celementvolumevariables.hh

dumux/geomechanics/el2p/el2pelementvolumevariables.hh

dumux/geomechanics/el2p/el2plocalresidual.hh

dumux/geomechanics/el2p/el2pproperties.hh

dumux/geomechanics/el2p/el2ppropertydefaults.hh

dumux/geomechanics/elastic/elasticproperties.hh

dumux/implicit/1p/1pproperties.hh

dumux/implicit/1p2c/1p2cproperties.hh

dumux/implicit/2p/2pproperties.hh

dumux/implicit/2p2c/2p2cproperties.hh

dumux/implicit/2pdfm/2pdfmproperties.hh

dumux/implicit/2pnc/2pncproperties.hh

dumux/implicit/3p/3pproperties.hh

dumux/implicit/3p3c/3p3cproperties.hh

dumux/implicit/box/assembler.hh

dumux/implicit/box/boxassembler.hh

dumux/implicit/box/boxelementboundarytypes.hh

dumux/implicit/box/boxelementvolumevariables.hh

dumux/implicit/box/boxfvelementgeometry.hh