Add diagnostic plot for field coverage

This MR addresses three things:

1. mocpy dependency has been relaxed to not require a version below 12 anymore.
2. the LoTSS DR2 MOC is now downloaded to the skymodels folder for better bookkeeping.
3. A diagnostic plot plots/field_coverage.png is now made to illustrate the field coverage as was suggested in #16 (closed). This is particularly useful if the starting model is LoTSS, where coverage can be incomplete.

An example of the generated plot:

Closes RAP-504