Feflow converter - use ogs6py

Purpose

With these changes, a coupling of the Feflow converter with ogs6py should be achieved. Instead of writing xml snippets, ogs6py creates a prj file that is not complete, but provides a good basis for creating an ogs simulation. Further entries must be made manually to complete the prj file.

Note

This MR should only be merged when MR!77 is merged. Since it is based on changes made to this MR. The coupling is introduced in the commit with the message "approach to couple ogs6py with feflow2ogs".