New Feature: Convert mapillary_image_description.json to GPX

[Uzver123]

Related to this feature request, but can be a separate feature that is useful for more flexibility.

For advanced editing scenarios (especially for GoPro Max footage that is lacking GPMF stream in converted rectilinear mp4 file on Windows platform) this could be very beneficial if you have command to generate .GPX file based on existing mapillary_image_description.json

[ptpt]

Make sense. It can be also useful for visualization.

[ptpt]

@Uzver123 Added in #759. Use the .gpx suffix to export GPX, e.g. --desc_path foobar.gpx