NWB file size for variable-shape ROIs

[ageorgou]

Can we reduce the file size when including variable-shape ROIs? Some issues and ideas:

• Pixel offsets duplicated across red and green channels:
  • Store in only one channel's Series, use reference (instead of full dataset) in the other's (not sure if possible?)
  • Store timestamps as another type of Series, have our ROISeries refer to them? (same uncertainty as above)
  • Use a new type to include both channels, but that would change the structure