[test] Unit test for VanGenuchten / Brooks-Corey material laws missing

We should test (for raw and regularized version):

• if derivatives match numerical derivatives
• unregularized same as regularized in the non-regularized range
• endPointPc() is the same as evaluation at Sw=1
• entryPressure is the same as evaluation at Sw=1
• test against some precomputed reference values (a good regression test)
• test eff to abs law

add more invariants that could be unit tested if you have an idea.