Utils for combining annotations

[sfmig]

Made an annotations_utils modules with functions to:

• combine several VIA JSON files into one (combine_multiple_via_jsons)
• generate a COCO-formatted annotation file from a VIA JSON file (convert_via_json_to_coco)

I ended removed the previous combine_coco function (which appended COCO files two-by-two) as I found combining VIA JSON files was easier (we just need to append the data, no need to keep track of the ids), and so we could write more easily a function that combines several files in one go.

The previous convert_coco function is the basis for the convert_via_json_to_coco function with minor additions.

Also included a minor fix to the frame extraction test which were failing before.

Closes #70

[nikk-nikaznan]

Looking very good! Good move to create the utils for this. Run this locally including the pytest. Apart from directory, seems fine