MOBT-666: Modification of scalar coordinate values within standardise

[bayliffe]

This PR adds the ability to modify scalar coordinate values, which are not time coordinates, within the standardise steps. The limitation to scalar and non-time coordinates are imposed out of fear of excessive decoupling between the data and the descriptors. The use case is envisaged as small tweaks to model independent values where these are deemed to be insignificant. Specifically alignment of the height coordinate for temperature forecasts between MO and ECMWF models which differ by 0.5m and are away from the surface.

Testing:

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

Acceptance test data PR: https://github.com/metoppv/improver_test_data/pull/45

[codecov[bot]]

Codecov Report

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

Project coverage is 98.40%. Comparing base (209507c) to head (13f49cb).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1984 +/- ## ======================================= Coverage 98.40% 98.40% ======================================= Files 124 124 Lines 12125 12138 +13 ======================================= + Hits 11931 11944 +13 Misses 194 194 ```

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