Documentation incomplete

[florianjehn]

• Docstrings do not follow our guidelines (https://github.com/allfed/ALLFED-Repository-Template)
• Incomplete readme with dead links
• Code is split between /src and notebooks. Notebooks should not be used to contain large chunks of code, as this makes things much harder to maintain.

[jamesmulhall]

I've merged our working branches over to main so these are all partially addressed now.

• Docstrings in /src are mostly compliant with pep8, but there are exceptions
• The readme point is finished and there is additional documentation under /docs.
• I have done my best to move all code for the main model to src and use scripts for implementing it. There are still some large-ish chunks of code in the data manipulation scripts, the mobility notebook, and the sensitivity notebook. Those last two have no direct impact on how the main model runs.

I'll leave open for now, but getting these to 100% is not currently a priority (unless you or someone else wants to request that)