Improve human readability in Python Generation

[FinbarArgus]

I have done this temporarily by postprocessing the generated python code.

See physiomelinks/circulatory_autogen#180 for more info

Generated files look like this:

3compartment.py
3compartment_utilities.py

This should be done with a libcellml template instead of doing it as a postprocessing... one day

[FinbarArgus]

I haven't yet done an analysis on the slowdown of using the StateView rather than directly accessing from a list... I expect it to be small