Split codebase intomolecularnodes and UI components

[BradyAJohnston]

The source of various problems we are running into is importing of modules / code for the add-on and for as a python module.

A potential solution for this is to release molecularnodes as a python module (which it currently is) but without any of the UI / operator code. That UI / operator code then is only included in the add-on build (and the molecularnodes python module will just be shipped with it as a .whl?) to be installed from the extensions platform.

Might lead to conflicting registering of properties, but in theory no extra classes need registering outside of the UI, just properties.