Matthieu Chourrout

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.

Hi, The `CloudVolumePrecomputed` class already has `mesh` and `skeletons` arguments but is lacking `segment_properties` described in [the neuroglancer precomputed format docs](https://github.com/google/neuroglancer/blob/master/src/datasource/precomputed/volume.md#info-json-file-specification). This would be a named argument with default value...