Behavior was made more consistent. Methods are now available
on all the (Atom|Residue|Chain)(Handle|View)s, including in
Python, and documented. A hash code is an `unsigned long`,
and requesting it on an invalid handle raises an `InvalidHande`
exception.
In practice, the following was implemented:
- Exported to Python for ChainHandles
- Added method on ResidueViews and ChainViews (including Python export)
- Raise InvalidHandle error on ResidueHandle (was: return 0)
- Explicitly check validity of ChainHandle (although behavior was to
  throw an error even without explicit check)
- Changed return value from `long` to `unsigned long` on
  Atom(View|Handle)s.

Fixes the SDF reader and documents the convention.

Also export the attributes where needed.