Bio.SVDSuperimposer cleanup to remove nested module name

This was prompted by having to fix an `import *` in this area of the code, and the recollection of epydoc complaining about the class and sub-module name clashing (one shadows the other making imports confusing).

This pull request essentially moves the code in `Bio/SVDSuperimposer/SVDSuperimposer.py` into `Bio/SVDSuperimposer/__init__.py` from where is was typically imported anyway.

As explained in the DEPRECATED file, this could break some existing code doing long-winded imports, but the standard import is not affected (and probably most users import it via Bio.PDB anyway and won't be affected at all).