2p box model: convert to the new structure

dumux/boxmodels/2p/2pboxjacobian.hh

dumux/boxmodels/2p/2pelementdata.hh

dumux/boxmodels/2p/2pfluidstate.hh

dumux/boxmodels/2p/2pfluxvars.hh

dumux/boxmodels/2p/2plocalresidual.hh

dumux/boxmodels/2p/2pmodel.hh

dumux/boxmodels/2p/2pnewtoncontroller.hh

dumux/boxmodels/2p/2pproblem.hh

dumux/boxmodels/2p/2pproperties.hh

dumux/boxmodels/2p/2psecondaryvars.hh

dumux/boxmodels/common/boxproperties.hh

test/boxmodels/2p/lensproblem.hh

test/boxmodels/2p/lensspatialparameters.hh

test/boxmodels/2p/test_2p.cc