Fix/materiallaw vangenuchten spline assert

Fixes #985 (closed). Spline construction might fail if the user input is thresholdswe = 1 (which was previously interpreted as just turning the regularization off). With this MR we support this again.

Looking for splines in brooks corey I saw that they are not used there but the documentation suggested so, so I updated the docs to match the code.

Supersedes !2458 (closed).