There should be a Python component to the oxmol wrapper

[thesketh]

At present, oxmol is a bare wrapper over molecule.rs. There should be a Python component to the wrapper, for three primary reasons:

• it will be much neater to handle errors on the Python side than on the Rust side, where to declare new error types we need to handle the GIL.
• The .so file generated with PyO3 doesn't provide us with a real package - we have a module which contains modules. This generates strange errors when trying to run e.g. import oxmol.spec.
• oxmol should eventually be listed on PyPI, and this is easier where we have the directory structure of a Python project

[thesketh]

RE: the three points:

• This is complicated by the fact that it's not possible to properly subclass the PyO3 classes, I'm unable to catch the errors from within Python. The error handling will have to be improved on the Rust side.
• This has been fixed by introducing the Python wrapper (and using Maturin for the build process)
• Done.

[thesketh]

I think PyO3/pyo3#947 needs to be resolved before I can make further progress on error and iterator handling.