Fix regridding issue where target grid is more than 2D

[bouweandela]

Description

In #968 a rechunking step was introduced if the target grid resolution was considerably higher than the source grid. However, it failed to take into account that the target grid could have more dimensions than just latitude and longitude. This pull request fixes the issue.

Closes #2085

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[codecov]

Codecov Report

Merging #2087 (ca2bb72) into main (92e20fd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2087   +/-   ##
=======================================
  Coverage   93.08%   93.08%           
=======================================
  Files         237      237           
  Lines       12785    12793    +8     
=======================================
+ Hits        11901    11909    +8     
  Misses        884      884           

Impacted Files	Coverage Δ
esmvalcore/preprocessor/_regrid.py	98.31% <100.00%> (+0.03%)	⬆️

[zklaus]

Just one little question. Otherwise looks good.

[valeriupredoi]

cheers, bud! A couple of minor points from me, but am requesting changes for @zklaus 's point to be considered (don't think he did review with req'd changes)

[valeriupredoi]

cheers, Bouwe 🍺