CoordinateExpand and CoordinateFlatten refactoring

[gemmaellen]

This request does the following:

• Removes the Flatten and Expand transforms from data.transform.coordinates in favour of writing these functionalities explicitly into the CoordinateFlatten operation in pipeline.operations/xarray and a new CoordinateExpand operation for reversal.
• Writes tests for these operations.
• Removes the ability to flatten multiple coordinates at once in favour of doing one thing at a time.
• Removes the SelectFlatten transform in favour of doing Select and then CoordinateFlatten in separate steps.
• Amends the documentation to point at the new arrangement.

[coveralls]

Pull Request Test Coverage Report for Build 18579479652

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 134 of 137 (97.81%) changed or added relevant lines in 3 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 61.335%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/pipeline/src/pyearthtools/pipeline/operations/xarray/reshape.py	61	64	95.31%

Files with Coverage Reduction	New Missed Lines	%
packages/data/src/pyearthtools/data/transforms/transform.py	2	53.71%

Totals
Change from base Build 18548575364:	0.2%
Covered Lines:	9535
Relevant Lines:	15127

---

💛 - Coveralls

[tennlee]

This looks great, thanks very much. A note for the future, we are hoping to remove or reduce the use of the BackwardsCompatibility decorator. At the moment we don't have too many dependants on the old patterns, so there's no need to re-create those or persist their use, unless they are actively relied upon in the codebase+notebooks. Changes look good, notebooks seem fine, tests look good. I will re-run the notebook but it's taking me a little time due to the data download proceeding slowly (and for reasons my cache version being unavailable right now).