util.coordinates: remove broken dist_to_coast function

[tovogt]

Changes proposed in this PR:

• Always use precomputed distances to coast in Centroids.set_dist_coast to avoid Persistent Wind Intensity Patterns in Historical Typhoon Tracks Beyond 180 Degrees Longitude #831 and similar issues with the implementation of util.coordinates.dist_to_coast (it is also quite slow, actually).
• Remove the util.coordinates.dist_to_coast function because it is now used nowhere.
• It also removes some helper functions that were only used in dist_to_coast, further reducing the amount of code in util.coordinates.

This PR fixes #831, see CLIMADA-project/climada_petals#106 for a follow-up PR in petals.

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[peanutfun]

Nice. While removing code is great, I would like to keep the public functions and add deprecation warnings. This way, we do not habe a major change following semantic versioning. I am fine with removing utm_zones though, seems like a really obsolete function to me.

[tovogt]

I added dist_to_coast, but would still opt to remove get_coastlines since it's clearly a helper function for the distance computations.

[emanuel-schmid]

Many thanks! I'd suggest to wait with merging though until 5.0 🤔

[tovogt]

Okay, this is ready to be merged.

@emanuel-schmid If we merge this before CLIMADA-project/climada_petals#122 is merged, we can also ditch CLIMADA-project/climada_petals#106 and instead include the necessary changes directly in CLIMADA-project/climada_petals#122. That's a bit easier, since CLIMADA-project/climada_petals#106 is only about a few lines anyway, and all of the affected lines will also be affected by CLIMADA-project/climada_petals#122