Updates following review comments

[gavinevans]

Addresses metoppv#2135

Description
This PR:

• Adds support for providing the max_features argument to the train QRF CLI.
• Adds warnings to aid monitoring.
• Provides updates to docstrings following the latest review of Implement Quantile Regression Random Forest metoppv/improver#2135.

Testing:

• Ran tests and they passed OK
• Added new tests for the new feature(s)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 14.81481% with 23 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (mobt_877_implement_qrf@1daa7d6). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...oad_and_train_quantile_regression_random_forest.py	8.33%	11 Missing ⚠️
...oad_and_apply_quantile_regression_random_forest.py	12.50%	7 Missing ⚠️
improver/calibration/__init__.py	28.57%	5 Missing ⚠️
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Additional details and impacted files

@@                    Coverage Diff                    @@
##             mobt_877_implement_qrf      #20   +/-   ##
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  Coverage                          ?   95.59%           
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  Files                             ?      142           
  Lines                             ?    14179           
  Branches                          ?        0           
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  Hits                              ?    13554           
  Misses                            ?      625           
  Partials                          ?        0           

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[Kat-90]

Technically it can be a string as well, but we aren't using that.