Add IndexRaster for gridding on reduced weighted rasters

[coroa]

IndexRaster holds (and is able to serialize) a reduced weighted raster, ie. one that is split in:

1. a flat indicator, where integer values from 1 to number of shapes indicate cells fully belonging to a single shape
2. a boundary made up of the weights similar to the previous weighted raster (but only covering cells actually on the boundary)

The boundary uses a MultiIndex called spatial as a sparse representation.

aggregate and grid methods allow to aggregate gridded data and to grid indexed data.

There is a from_weighted_raster constructor which transforms a weighted raster into the reduced form. It would be good to add a rasterization strategy similar to maybe hybrid which directly creates an IndexRaster.

Documentation and testing has not been addressed yet.

Two new dependencies need to be pulled in (we could also make them optional?):

1. flox is necessary for aggregating data from the grid efficiently
2. cf_xarray provides two convenience functions which allow serializing and deserializing multi-index coordinates

[coroa]

Ok, was able to fix the failing cf_xarray import by bumping the python version to 3.9, but there are some regression tests failures i am not getting to the bottom of today.

[gidden]

Hi @coroa - I will do a more thorough review when I have time later today. But I just ran the tests locally and all pass - so have restarted the job here.

[gidden]

Hi @coroa I have now reviewed and can only admire the approach and implementation. Let me know when you have a chance to update the docs and add tests, which will also help me do a full review.

[gidden]

Hey @coroa I just realized here that everything in this PR is a pure addition and no existing code is touched. So I have to say I'm really confused as to why the tests are failing (again noting they pass on my local machine).

[gidden]

rebased on main

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 38.02817% with 44 lines in your changes are missing coverage. Please review.

Project coverage is 68.25%. Comparing base (3d07c4f) to head (569947c).

Files	Patch %	Lines
src/ptolemy/raster.py	38.02%	44 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #10      +/-   ##
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- Coverage   76.73%   68.25%   -8.49%     
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  Files           2        2              
  Lines         245      315      +70     
  Branches       57       74      +17     
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+ Hits          188      215      +27     
- Misses         26       69      +43     
  Partials       31       31              

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[gidden]

Current test failures is because of black. Todos left for this PR are:

1. add tests
2. change linting to ruff

[coroa]

Ready for review! And merging.

[gidden]

Looks great @coroa - I did a scan of the doc notebook and tests, which all make sense to me. Thanks so much for this!