Support parallel processing within DistanceTo plugin

[gavinevans]

Related to https://github.com/metoppv/mo-blue-team/issues/934

Description
This PR adds the capability to generate a "DistanceTo" ancillary using parallel processing to speed up the generation of this form of ancillary.

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 95.11%. Comparing base (84a8944) to head (e1bdc4e).
⚠️ Report is 131 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2220      +/-   ##
==========================================
- Coverage   98.39%   95.11%   -3.28%     
==========================================
  Files         124      147      +23     
  Lines       12212    14843    +2631     
==========================================
+ Hits        12016    14118    +2102     
- Misses        196      725     +529     

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[maxwhitemet]

Happy with the changes made. Approved 👍