Ensure objects containing Cubes are saved as netCDF files

[gavinevans]

Addresses metoppv#2135

Description
This PR:

• Updates the CloudCondensationLevel and GradientBetweenAdjacentGridSquares to return a CubeList of cubes, rather than a tuple of cubes.
• Updates cli/__init__.py to support the saving of an iterable containing cubes to a netCDF file, rather than a pickle file, in case there is confusion in the future if e.g. a tuple of cubes gets saved as a pickle file, rather than a netCDF file.

There are no unit tests or acceptance test failures:
== 11630 passed, 482 skipped, 1 xpassed, 11271 warnings in 134.14s (0:02:14) ===

Testing:

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

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.58%. Comparing base (1d9ad86) to head (6414431).
⚠️ Report is 1 commits behind head on mobt_877_implement_qrf.
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Additional details and impacted files

@@                   Coverage Diff                   @@
##           mobt_877_implement_qrf      #24   +/-   ##
=======================================================
  Coverage                   95.58%   95.58%           
=======================================================
  Files                         142      142           
  Lines                       14180    14180           
=======================================================
  Hits                        13554    13554           
  Misses                        626      626           

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