Merged requested to merge fix/dkrn-dswe-brookscorey into master
Fix wrong implementation of the analytical derivative of
dkrn_dswe in Brooks-Corey and Van Genuchten.
Fixes #844 (closed)
Tests are implemented in !1874 (merged). Once everything is set there this is mostly likely well-tested in the new unit test added with !1874 (merged). However, we have to merge this first and then merge the tests in !1874 (merged) after.