Uniform Python interface to C++ library

[unzvfu]

The C++ library is currently accessed through the interface generated by Python's CFFI, which is clumsy and not idiomatic. It would neater if we wrapped interaction with the C++ library completely in a more Pythony way. This ought to complement issue #66 which is about avoiding copying back and forth from the Python runtime.

Aha! Link: https://csiro.aha.io/features/ANONLINK-69