spline: add (optional) extrapolation in the eval and eval-derivative methods

splines are extrapolated attaching straight lines at the spline's
endpoints which exhibits the same value and derivative as the spline
at the end points. this makes the eval() and evalDerivative() methods
more covenient to use.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6053 2fb0f335-1f38-0410-981e-8018bf24f1b0