MOBT-797: Custom cell method addition during thresholding

[bayliffe]

Add a test to demonstrate the application of a custom cell method as part of coordinate collapse when using the threshold CLI.

The resulting cube has the following metadata:

probability_of_air_temperature_above_threshold / (1) (projection_y_coordinate: 100; projection_x_coordinate: 100)
    Dimension coordinates:
        projection_y_coordinate                                              x                             -
        projection_x_coordinate                                              -                             x
    Scalar coordinates:
        air_temperature                              280.0 K
        forecast_period                              0 seconds
        forecast_reference_time                      2017-02-01 03:00:00
        height                                       1.5 m
        time                                         2017-02-01 03:00:00
    Cell methods:
        0                                            realization: mean (of air_temperature)
    Attributes:
        Conventions                                  'CF-1.7'
        institution                                  'Met Office'
        mosg__model_configuration                    'uk_ens'
        source                                       'Met Office Unified Model'
        title                                        'Post-Processed Operational MOGREPS-UK Model Forecast'

[Katie-Howard]

I've looked through this data and its looks sensible to me 👍