Change check_footprint.py to fit structuring of check_symbol.py

This changes the structuring of check_footprint.py from function to class based to make it similar to check_symbol.py. With these changes the check functions can be imported with a single import and class declaration, many of the initializations done in the main of the old check_footprint.py don't have to be copied over to other uses of the checker anymore.

My motivation for this was an easier integration of the check functionalities in the procedures involved in the release procedures for libraries and PCBs in my company.