Recursive filter mask zeros modifications

[bayliffe]

This PR:

• Adds mask_zeros option for recursive filter to the associated CLI
• Modifies the order of the zeroing application in the plugin such that the variable_mask option still has some meaning in the context of zeroing data. Without this change variable_mask will always have to be set when using mask_zeroing as zero values in the data may appear in different locations in different slices.
• Adds an acceptance test for the mask_zeroing.

New acceptance test data: metoppv/improver_test_data#109

Testing:

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.68%. Comparing base (84a8944) to head (63be5d7).
⚠️ Report is 116 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2199      +/-   ##
==========================================
- Coverage   98.39%   95.68%   -2.72%     
==========================================
  Files         124      146      +22     
  Lines       12212    14519    +2307     
==========================================
+ Hits        12016    13892    +1876     
- Misses        196      627     +431     

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[brhooper]

Thanks @bayliffe, this change looks sensible to me.

[gavinevans]

Thanks @bayliffe 👍

I've suggested fixing one typo.