Support non-constant frame rate

[AmitMY]

We have had a few cases by now of non constant frame rate videos, e.g. #127

I would like to add support for this. When creating a pose file, we should detect this kind of frame rate, and support a 0 fps which means fluctuating, followed by the number of total frames (which we already have) followed by exact frame timing array. That way, when we load the data based on timing, we can tell from which frame until which frame to load.