Added a cell method to indicate an accumulation for freezing rain where non-instantaneous

[maxwhitemet]

Added a cell method to indicate an accumulation for freezing rain where non-instantaneous.

Addresses: #599

Testing:

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

CLA

• If a new developer, signed up to CLA

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.40%. Comparing base (84a8944) to head (615af5a).
Report is 54 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #2062    +/-   ##
========================================
  Coverage   98.39%   98.40%            
========================================
  Files         124      134    +10     
  Lines       12212    13141   +929     
========================================
+ Hits        12016    12931   +915     
- Misses        196      210    +14     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[brhooper]

Thanks @maxwhitemet, this seems to be a concise and easily understandable change. I've added one comment requesting a change to the code.

When running the acceptance tests I've encountered 2 failures caused by cell methods existing on the result cube but (obviously) not being on the kgo. I should have predicted that this would be an issue and discussed it with you. We can have a chat about how to rectify this, as I don't think we have spoken in detail about the acceptance tests before.

[brhooper]

Thanks for these changes @maxwhitemet, I have added one further comment (which I probably should have added the first time I reviewed this - sorry!).

[brhooper]

Thanks @maxwhitemet, I'm happy this into second review now, though I think there are 2 small things to tidy up before merging.

1. You currently have 2 PRs going for the test data (updated freezing_rain KGOs to match changes to cell method improver_test_data#65 and Updated KGOs for freezing-rain to include a cell method improver_test_data#66). One of these can be closed.
2. I think that you will need to pull the latest version of master into this PR as a fix for the Github action failures has been merged (conda-forge sphinx doc build failure CI #2070). We can discuss how to do this when you are free.