improved inverse iteration dofpt3. Rüdigers test case now converges, and there are no errors left in ticket in #964. The iteration was actually unrelated to the original ticket title, but came up in the same thread. 

git-svn-id: https://svn.modelica.org/projects/Modelica/trunk@6066 7ce873d0-865f-4ce7-a662-4bb36ea78beb