Split up spatial_ref module into feature-focused submodules.

[metasim]

• I agree to follow the project's code of conduct.
• I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

---

Also added small bits of documentation and fixed clippy warnings.

Aside: I originally started this refactor because I'm about to expose the GCP API and erroneously thought it would be a part of the spatial_ref API. But now I see it's only raster-related. Therefore, this is basically some simple house-keeping...

[lnicola]

I didn't look at the other changes, but I don't think that function should be unsafe. I filed #360 for that, which.. you've already seen? 😄

[metasim]

@lnicola

I didn't look at the other changes, but I don't think that function should be unsafe. I filed #360 for that, which.. you've already seen? 😄

Ah! I misread your ticket!... and thought you were saying the opposite 🤦

I defer to your experience here. I'll remove it here, and we can create a new PR to handle the other cases.... or find an all around better way.

[lnicola]

Not sure if users will care about this :-).

[lnicola]

Thanks for these refactoring and documentation PRs, I think they're great!

[metasim]

bors r+

[bors]

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• ci gdal-32
• ci gdal-32 (--all-features)
• ci gdal-33
• ci gdal-33 (--all-features)
• ci gdal-34
• ci gdal-34 (--all-features)
• ci gdal-35
• ci gdal-35 (--all-features)
• ci ubuntu-lts
• ci ubuntu-lts (--all-features)