support reading metadata from class score files

[joefutrelle]

Currently, support for accessing class scores for score format versions 2 and 3 (HDF5) ignores metadata and all fields in the h5 file that aren't class scores or class labels.

This feature would provide that metadata so that it can be consumed by tools such as the IFCB dashboard.

[joefutrelle]

The relevant, useful fields in the data I've looked at are /metadata/model_id and /metadata/timestamp. /metadata/version refers to the version of the output format being used.