MOBT-689 (Pt2): Spot forecast subsetting using a neighbour cube

[bayliffe]

Allows a user to subset an existing spot-forecast via the spot-extraction CLI using a neighbour cube. This will allow us to apply a verification neighbour cube to the end-of-chain forecasts in the preverspot steps to produce a much smaller and controlled set of sites for verification. It removes the need for the preverspot step in the suite to be aliased to different commands for different steps in the chain.

This subsetting is performed using a coordinate selected by the user, with the doc-strings indicating this should be an ID coordinate (e.g. wmo_id or met_office_site_id), rather than e.g. altitude. I have not designed this to handle float comparisons, nor constraining on multiple coordinates as would be required if sites were to be extracted using latitude + longitude.

Acceptance test data: https://github.com/metoppv/improver_test_data/pull/46

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 98.42%. Comparing base (84a8944) to head (8fa5478). Report is 9 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2002 +/- ## ========================================== + Coverage 98.39% 98.42% +0.02% ========================================== Files 124 126 +2 Lines 12212 12417 +205 ========================================== + Hits 12016 12221 +205 Misses 196 196 ```

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