optimized `operator` module

This merge request supercedes !27 (closed). The rewrite in Cython required some significant refactoring, to the point that looking at incremental diffs becomes more of a hinderance than a help.

A few comments with respect to the previous version:

• The reduce keyword has been reduced: the user can explicitly reduce if they want to, (this is a cheap operation)

The following need to be implemented before this is ready to be merged

• Switch to pytest (possible now that we are rebased on master)
• Rename LocalObservable to _LocalOperatorBase and make it non-public. Rename Charge to LocalOperator. Rewrite docstrings to remove references to "observables"
• Remove SPARSE operation and associated stubs, remove uneeded arguments from _operate
• Refactor MatrixArray into BlockSparseMatrix that contain: offset data, shape data, matrix value data. This will prevent us from re-calculating orbitals

closes #14 (closed).