Process Garmin .FIT files

[zlavergne]

When using the new Garmin Virb Ultra 30 in Video Timelapse capture mode, the camera no longer creates a single .GPX file for each .MP4 like the older versions of the camera did. It now creates one .FIT file for the whole time that the camera is on.

I am migrating from the older Virb to these new ones and need to process .FIT files for GPS information. I have already started making this outside of the mapillary_tools but I it would make execution simpler for me if the mapillary_tools housed the parsing of the .FIT files.

The process requires the fitparse library but otherwise mimics how mapillary_tools processes gpx or exif data.

I can have an initial PR ready by next week if you're interested.
cc @cookry