Utilities for sorting spatialdata points along z-order curve

[keller-mark]

Towards scverse/spatialdata#974

Implementing here in vitessce.data_utils initially to be able to test things out

TODO:

• Fix rounding issues in test
• Provide large (or small) example points dataset in unit test (crop/filter current test fixture?)
• Store feature_key codes column name in metadata
• Store bounding box metadata via parquet metadata (rather than sdata.attrs)?

[keller-mark]

Note the custom .attrs for the point element are not written to disk. See https://github.com/scverse/spatialdata/blob/aeef0cc3ebd7c10a7ed17b84415213bb259a5f4b/src/spatialdata/_io/format.py#L116 Need to either make a PR to spatialdata or store the attributes somewhere else (e.g., the parquet schema metadata?)

For now/testing, I have manually edited the .zattrs file to include the bounding_box property: