Improved handling of weights in MLR diagnostics

[schlunma]

Description

• Change the default value of weighted_samples for MLR diagnostics: If weighted_samples is not given, do not use weights.
• Raise error if weights cannot be calculated but are requested by the user.
• Update API doc of the functions that calculate weights.

• Closes Use of sample weights in MLR diagnostics can be confusing #2624
• Link to documentation:
  • https://esmvaltool--2625.org.readthedocs.build/en/2625/api/esmvaltool.diag_scripts.mlr/init.html
  • https://esmvaltool--2625.org.readthedocs.build/en/2625/api/esmvaltool.diag_scripts.mlr/models.html
  • https://esmvaltool--2625.org.readthedocs.build/en/2625/api/esmvaltool.diag_scripts.mlr/postprocess.html
  • https://esmvaltool--2625.org.readthedocs.build/en/2625/api/esmvaltool.diag_scripts.mlr/preprocess.html

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[schlunma]

@valeriupredoi Would you mind giving this a brief technical review?

This basically only improves the documentation and error messages. I successfully tested all affected recipes; the output of them is the same as for the main branch. Thanks!!

[valeriupredoi]

looking nice, cheers, dude! Just a couple very minor wiggles from me 🍺

[schlunma]

Cheers, thanks V.!!