Refactor plot.py

### What problem does your feature request solve?

Every time I look at `src/CSET/operators/plot.py` I get a headache. It is by far the most complicated file we have, with a lot of stuff in it. Reviewing changes to it is painful and I'm not that confident in its testing due to all the conditional code.

This can be backed up empirically, as plot.py is our single most-changed file in CSET, with over three times the number of changes that the next most changed python file (read.py) has had. This indicates to me that we don't have it right yet.

### Describe the solution you'd like

We should refactor it somehow. TBD exactly how we can improve it, but I'm creating this issue to give some motivation to look into it.

### Describe alternatives you've considered

It does work currently, but I fear that further maintenance and improvements are going to become more difficult.

Start by reading src/CSET/operators/plot.py and reviewing its conditional paths and change history. Identify concrete responsibilities that could be separated and the tests covering them. Done would require an agreed refactoring plan and preserving current plotting behavior, but this issue does not define the target structure or acceptance criteria.

Written by the indexing model from the issue text.