Enable site cube input to ConstructReliabilityCalibrationTables

[lucyleeow]

First part of the work to add site & 'point_by_point' functionality to reliability calibration (as detailed elsewhere), similar to what's been done in EMOS: #1170. Work will be split into separate PRs to aid reviewing.

Enable ConstructReliabilityCalibrationTables to accept both grid and site cubes as input forecast and truths. Proposed format of reliability table for site cube:

  reliability_calibration_table / (1) (air_temperature: 2; table_row_index: 3; probability_bin: 5; spot_index: xx)
       Dimension coordinates:
            air_temperature                           x                -               -                -                             
            table_row_index                           -                x               -                -                             
            probability_bin                           -                -               x                -                             
            spot_index                                -                -               -                x                             
       Auxiliary coordinates:
            table_row_name                            -                x               -                -                             
            latitude                                  -                -               -                x
            longitude                                 -                -               -                x
            wmo_ids                                   -                -               -                x
       Scalar coordinates:
            forecast_reference_time: xx
            forecast_period: 68400 seconds

Acceptance criteria:

• ConstructReliabilityCalibrationTables accepts site cubes as well as grid cubes
• Current tests pass
• Add tests for site functionality

ping @gavinevans @bayliffe since you seem to have worked the most on reliability calibration. Please let me know of any questions/concerns.

cc @anja-bom @tjtg