Allowed ignoring of scalar time coords in multi_model_statistics

[schlunma]

Description

This PR makes the usage of the option ignore_scalar_coords=True in multi_model_statistics work for scalar time coordinates.

Closes #1960

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[codecov]

Codecov Report

Merging #1961 (c93141d) into main (5593c1a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1961   +/-   ##
=======================================
  Coverage   92.79%   92.79%           
=======================================
  Files         236      236           
  Lines       12476    12484    +8     
=======================================
+ Hits        11577    11585    +8     
  Misses        899      899           

Impacted Files	Coverage Δ
esmvalcore/preprocessor/_multimodel.py	97.94% <100.00%> (+0.05%)	⬆️

[valeriupredoi]

looking good, cheers Manu! Just a couple minor points from me 🍺