Separating cross-contour notebook

[wghuneke]

Notebook is cleaned up, code that extracts the contour is moved to a new notebook.

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[adele-morrison]

I did a bunch of edits on Approach 2 in Create_contour.ipynb.

Based on decisions today at the hackathon, we still need to remove the creation of transport masks for Approach 1 in Create_contour.ipynb and just simplify to produce a mask to e.g. plot temperature on the shelf. When we do that, we can also close #383, because it will no longer be relevant.

[wghuneke]

I have now cleaned up Approach 1 and removed code that calculates the transport masks.

I have also checked the saved files we reference at the beginning. I did this by loading the masks and inspecting contour_masked_above. If that shows diagonal connections, I interpreted that as being produced using Approach 1. Does this sound reasonable?