Addresses metadata issues for duration subdivision plugin

[bayliffe]

• Modify duration subdivision plugin to correctly recalculate the forecast periods for the shorter periods that are created.
• Modify the netcdf save code to explicitly exclude the inclusion of any duplicate cell methods.

Testing:

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

Updated acceptance test data:
metoppv/improver_test_data#74

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.41%. Comparing base (84a8944) to head (cf9239b).
Report is 73 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2094      +/-   ##
==========================================
+ Coverage   98.39%   98.41%   +0.01%     
==========================================
  Files         124      135      +11     
  Lines       12212    13309    +1097     
==========================================
+ Hits        12016    13098    +1082     
- Misses        196      211      +15     

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[brhooper]

Thanks @bayliffe. This meets the acceptance criteria and I don't have any review comments, so happy to approve.

[gavinevans]

Thanks @bayliffe 👍

I've just noticed one typo.