Separate tracking and file handling logic

At the moment there's a single class that does everything. To isolate the tracking from the filenames and other logic then an alternative approach could be:

One class that is passed the time period, filenames, etc. that then calls TempestExtremes.

An outer class that's specific to the UM that handles the logic of the filenames and what period to track.

An extra intermediate class could be one that does the regridding and generates the files that are required by the small class that calls TempestExtremes.

Start by locating the single class that currently handles tracking, filenames, regridding, and TempestExtremes invocation. Read how the time period and generated files flow through it, then define the class boundaries described in the issue. Done means tracking is isolated from UM-specific filename and period logic, with regridding and required-file generation separated where appropriate.

Written by the indexing model from the issue text.