Clean up model_kwargs

[lewisblake]

Is your feature request related to a problem? Please describe.
model_kwargs appears in three different places in a config and it's very confusing what the differences are. It appears in colocation_opts, in ModelEntry instances, and in ModelEntry.kwargs.

Describe the solution you would like to see

model_kwargs should only exist at in one place and all parts of the code which need it should reference that one place.

• Would you be able to work on this solution yourself? Yes, but I would like to involve @dulte on these disucssions as well. We've started working a bit on this.
• How can the Pyaerocom development team assist you? NA

Additional context
#1381
#1184