regularized brooks corey: change back to splines

for saturations > 1 limit the relative permeability to 100%. this avoids
negative saturations if some residual saturation is used

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@4293 2fb0f335-1f38-0410-981e-8018bf24f1b0