{Spline,PiecewiseLinear}TwoPhaseMaterial: limit the relperms to [0, 1]

it used to be the range of the sampling points. If for some reason the
sampling points describe a subset of the full range, the material laws
use extrapolation outside of the table and properly limit the values
of that operation.

In the real world, the effect of this change is rather limited because
all tested tables used the range...

TODO: adapt the evalDerivative() methods. This is not very urgent, though
because they are usually not used...