Added cumulative sum preprocessor

[schlunma]

Description

This PR adds the preprocessor cumulative_sum, which calculates cumulative sums along a given coordinate. Corresponding numpy function: np.cumsum.

Example:

preprocessors:
  preproc_cumulative_sum:
    cumulative_sum:
      coord: time
      weights: true

@bouweandela This is necessary for a REF diagnostic which I am currently developing (TCRE), so it would be great to have this in v2.12.0!

Link to documentation: https://esmvaltool--2642.org.readthedocs.build/projects/ESMValCore/en/2642/recipe/preprocessor.html#cumulative-sum

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.99%. Comparing base (31fe0d4) to head (35703ef).
Report is 52 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2642   +/-   ##
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  Coverage   94.99%   94.99%           
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  Files         255      255           
  Lines       14793    14824   +31     
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+ Hits        14052    14082   +30     
- Misses        741      742    +1     

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[bouweandela]

Looks very nice already @schlunma! I left a few questions.