changed velocity output of the 2p2cmodel: both phase velocities are

written out as VectorField, if VELOCITY_OUTPUT is set. As soon as the
ParameterTree is implemented properly in the boxmodels, this could
possibly be selected in the parameter file.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6082 2fb0f335-1f38-0410-981e-8018bf24f1b0