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.

GetPrev() or GetNext() may return invalid residue handles. It is
unclear what happens next, find() may simply (hopefully) find nothing,
however this feels unsafe. This fix guards against this possibility
by checking for handles validity before giving them to find().

Fixes the SDF reader and documents the convention.

Also export the attributes where needed.

Many of them remain, as they're triggered by Boost code too. Specifically
when importing boost python. Relevant Github discussion:
https://github.com/boostorg/python/issues/359