Fix wrong implementation of dkrn_dswe in Brooks-Corey and Van Genuchten
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.
Edited by Timo Koch