[2cstokes2p2c]

changed the way how, the boundaryLayerModel parameter is set; now it is not
mandatory to specify it in the input file, but it is disabled by
default


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@13886 2fb0f335-1f38-0410-981e-8018bf24f1b0