SnowSplitter output cube names are now the same as their StaGE couterparts

[MoseleyS]

Addresses https://metoffice.atlassian.net/browse/EPPT-650

While testing the EPP Precipitation Type workflows, I have found that the output cube names for rain/snow rate/accum data are not the same as the same data found on StaGE. For accumulation data, the rain and snow outputs had the names of the precipitation cube. This PR modifies the new cube name generation, which is generated from a precipitation cube name, so that:

• "precipitation" is replaced with either "rainfall" or "snowfall" (not "rain" or "snow")
• "lwe_", which means liquid water equivalent, is removed if the output is rain.

Testing:

• [x] Ran tests and they passed OK
• [x] Updated KGOs: https://github.com/metoppv/improver_test_data/pull/42
• [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:

Comparison is base (c43ec0a) 97.15% compared to head (0e3cdc2) 98.40%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1975 +/- ## ========================================== + Coverage 97.15% 98.40% +1.24% ========================================== Files 124 124 Lines 12065 12069 +4 ========================================== + Hits 11722 11876 +154 + Misses 343 193 -150 ```

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[MoseleyS]

These changes look fine, however I think additional changes are required to the acceptance test data to accommodate them. At the moment this PR causes the snow_splitter CLI acceptance tests to fail as the kgo cubes have incorrect cube names.

Oh dear. I must have been falling asleep to miss this. Acceptance Test PR added: https://github.com/metoppv/improver_test_data/pull/42