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.