Fixes problem with crop-tgrid

The function xr_crop_tgrid_data was failing tests. The issue has been identified and fixed. This implements the fix from !25.

marked this merge request as draft

added 3 commits

• bc11f16b - updated tests, nabla failing
• a4997410 - passing all tests
• d8146433 - added the tgrid docstring example to the tests

Compare with previous version

added 2 commits

• 701a6776 - a test to ensure the docs are building
• ebcbe52d - ran black on tests

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I have now added extra tests from !25 and a fix for the nabla function.

marked this merge request as ready

Looking good! Thank you, Fraser!

mentioned in commit 33c05928

merged