Overhaul "differential slewing"

The refactor of the rotator module had some impact in the align module. There was a report about problems with a 10Micron mount when using "differential slewing". A subsequent merge did not solve the problem. A long thread analyzes the issues with "differential slewing" with a Paramount and TSX. This merge request resolves these problems. The option "differential slewing" is now integrated into the adapted align module and works similar to the standard mode ("sync slewing") of the align module. Among other things "differential slewing" can now approach the object with several iteration steps if necessary.