Add B&C WRM, allow magic omega rel perm model to use B&C lambda

[ecoon]

We're starting to use more models that require Clapp & Hornberger b, so it may be useful to use Brooks & Corey, which has a parameter closely related to b.

Also, we want to use this in rel perm "magic omega" model from Niu and Yang which allows more infiltration into freezing soils.

Change that model to allow spatially variable b, add B&C model back in, and test with both.