Factor out transformations for QRF

[gavinevans]

Addresses metoppv#2135

Description
This PR factors out the apply_transformation code into a function and adds leniency for unit test results when NaNs are involved.

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 10.00000% with 9 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (mobt_877_implement_qrf@cecd99e). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...r/calibration/quantile_regression_random_forest.py	10.00%	9 Missing ⚠️
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Additional details and impacted files

@@                    Coverage Diff                    @@
##             mobt_877_implement_qrf      #18   +/-   ##
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  Coverage                          ?   95.69%           
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  Files                             ?      142           
  Lines                             ?    14162           
  Branches                          ?        0           
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  Hits                              ?    13552           
  Misses                            ?      610           
  Partials                          ?        0           

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