MOBT-430: Synthetic data utilities for creating spot-cubes

[bayliffe]

Addresses https://github.com/metoppv/mo-blue-team/issues/430

• Adds the following helper functions for use in creating unit tests that use spot-cubes:
  • set_up_spot_variable_cube
  • set_up_spot_percentile_cube
  • set_up_spot_probability_cube
• Adds associated unit tests
• Modifies calls to the existing gridded functions where these did not use the keyword arguments explicitly.
• Modifies some unit tests as an example of how this can be used.
• Centralises the date-time format "%Y%m%dT%H%MZ" as we use this in several places and have never done so.

The commits are broken up as follows:

1. new functions, tests, fix-up existing calls, and (sorry) a partially modified choose weights test.
2. centralise the date-time format
3. modify some unit tests as examples of usage.

Testing:

• [x] Ran tests and they passed OK
• [x] Added new tests for the new feature(s)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.40%. Comparing base (84a8944) to head (93d5760). Report is 4 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1993 +/- ## ========================================== + Coverage 98.39% 98.40% +0.01% ========================================== Files 124 124 Lines 12212 12322 +110 ========================================== + Hits 12016 12126 +110 Misses 196 196 ```

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