refactored extents algorithm

[erialC-P]

This PR adds in the refactored extents algorithm to our production code workflow.

I have:

• replaced the contents of the former extents.py with the new functions and workflow prototyped in notebooks/experimental/Extents_experiments.ipynb
• moved the function ocean_mask from the former extents.py script into elevation.py
• unfortunately, this may have broken the application of the ocean_mask func in the elevation function. For now, I have defaulted back to 'don't ocean mask' in the elevation call in the CLI function to get around this. I believe that this masking will be replaced by the new connectivity workflow in a future PR which is why I didn't investigate the bug further.
• I also did not create functionality to return the connectivity mask as part of this workflow, believing that might be more easily achieved in a future PR when connectivity is built into the elevation function.

Following this PR, I think we're ready to either:

• a) conduct a broadscale extents test on a select series of tiles or;
• b) move the connectivity function out of extents.py and into the elevation function in elevation.py prior to broadscale testing.

[github-actions]

For full integration test results, refer to the Tests directory README.