Expose getFramesAtIndices and getFramesDisplayedByTimestamps as public Python methods of VideoDecoder

[NicolasHug]

This is about exposing the 2 C++ methods from #280 and #282 into the Python class.

It feels natural to want these methods to be called get_frames_at() and get_frames_displayed_at(), but these names are already occupied by our "range" function. We may then need to rename these existing "range" functions (see diff).

[scotts]

I was puzzling over this myself, and I like this solution. I had considered keeping all current APIs as-is, and adding get_frames_at_indices() and get_frames_displayed_at_timestamps(), which is clunkier. This is cleaner.

[ahmadsharif1]

This is not my first preference, but you can merge it.

Reason is that "displayed" is not the critical difference b/w the two methods. Frames are always displayed -- whether at an index or timestamp.

I would have picked get_frame_at_index and get_frame_at_time, get_frames_at_index_range, get_frames_at_time_range, etc.

I like names that you can read from the callsite to tell what parameters they would need and what they are doing and other people may disagree with that.

[NicolasHug]

Thank for the feedback. I'll follow-up with separate PRs.