Refactor dbm file structure

Move classes from ~6000LOC models/dbm/__init__.py to own files.

API, including imports stays the same and should not break any existing code.

Cave: It has a cherry-picked "Fix CI" bfc3aa8f1 from PR https://github.com/lisa-lab/pylearn2/pull/563. This was to see, whether CI tests still pass. They do. In case the other PR won't be pulled, I'm happy to revert the "Fix CI" commit here.

Cheers
~markus