Add segment_prop_dir and segment_properties arguments for the precomputed format

[chourroutm]

This PR adds a new argument to handle the "segment_properties" sidecar subdirectory and its info file.

References:

• https://github.com/google/neuroglancer/blob/master/src/datasource/precomputed/volume.md#info-json-file-specification
• https://github.com/google/neuroglancer/blob/master/src/datasource/precomputed/segment_properties.md

This will solve #637.

[william-silversmith]

Hi! This is great, thank you! Is the PR ready for review? I was looking it over and it didn't seem to be actually doing anything with the segment_properties yet. Thanks again, this will be a great contribution!

[chourroutm]

Hi! At the moment, it only adds the "segment_properties" field to the info file, to point to the subdir with the data. I'll have to write a CSV-to-segment_properties snippet anyway, which I can append to the PR. Should I withdraw the PR for now?

Also, I cannot git clone the files onto my Windows 11 workstation because of some filenames which contain ':'.