Add attributes to ImageSeries

[rly]

As per discussion with NWB + CatalystNeuro team.

Add num_frames_per_external_file to ImageSeries. There is currently no way to figure this out without opening the image files.

This would help support a starting_time + sampling_rate method for time.

Also consider adding a way to allow adding a video file and not having it start at the first frame. This use case came up in Loren Frank's lab yesterday but is not a big deal if it is not supported.

[CodyCBakerPhD]

Also consider adding a way to allow adding a video file and not having it start at the first frame. This use case came up in Loren Frank's lab yesterday but is not a big deal if it is not supported.

Sounds like instead of just num_frames_per_external_file or starting_frames, this could become frame_index_range which is a list of length-2 tuples, each tuple being start frame index and end frame index respectively. If using entire video, this would become (0, num_frames_in_that_video)

Also seems easiest to implement this in a separate neurodata type such as ExternalImageSeries (though it's annoying how the 'l' in 'External' looks so similar to the 'I' in 'Image'..., depending on text renderer)