Area average time series

[mo-tomosevans]

Include area average time series plots capability.
Addresses #1910.

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[github-actions]

[mo-tomosevans]

The image attached was generated for air temperature, having used the AREA_AVG method. This is for one cycle, not yet introduced a method/recipe for running over multiple data file inputs.

[mo-tomosevans]

Having difficulty introducing existing aggregation methods to create aggregated area average plots.
When file path is pointing to operational UKV data at <path>/*.pp, ensure_aggregatable_cube does not produce an aggregated cube and fails, outputting

ValueError: ("Cube should have 'forecast_period' and 'forecast_reference_time' dimension coordinates."

I tried the pre-existing generic_surface_domain_mean_time_series_case_aggregation_*.yaml recipes with the same data and they all produced the above error, so I think it may be part of a wider issue for how the archived ukv data is loaded in read_cubes.