[folder structure] move and rename files from implicit/common, adapt includes

dumux/freeflow/stokes/stokeslocaljacobian.hh

dumux/freeflow/stokes/stokeslocalresidual.hh

dumux/freeflow/stokes/stokesmodel.hh

dumux/freeflow/stokes/stokesproblem.hh

dumux/freeflow/stokes/stokesvolumevariables.hh

dumux/geomechanics/el1p2c/el1p2clocaljacobian.hh

dumux/geomechanics/el1p2c/el1p2cvolumevariables.hh

dumux/geomechanics/el2p/el2pbasemodel.hh

dumux/geomechanics/el2p/el2pfluxvariables.hh

dumux/geomechanics/el2p/el2plocaljacobian.hh

dumux/geomechanics/elastic/elasticvolumevariables.hh

dumux/implicit/1p/1pmodel.hh

dumux/implicit/1p/1ppropertydefaults.hh

dumux/implicit/1p/1pvolumevariables.hh

dumux/implicit/1p2c/1p2cmodel.hh

dumux/implicit/1p2c/1p2cvolumevariables.hh

dumux/implicit/2p/2pmodel.hh

dumux/implicit/2p/2ppropertydefaults.hh

dumux/implicit/2p/2pvolumevariables.hh

dumux/implicit/2p2c/2p2cmodel.hh