Interface between loaders and `GtsfmData`

[travisdriver]

this seems like this is a more general purpose function that should go into GtsfmData or something, like

class GtsfmData:
     @classmethod
     def from_colmap_model(cls, colmap_model_dir: str) -> "GtsfmData":

would that make sense?

Originally posted by @johnwlambert in https://github.com/borglab/gtsfm/pull/258#discussion_r701511870

[travisdriver]

I think this is a good idea. However, I think the GtsfmData class needs to be restructured slightly for the loaders to be able to utilize it (e.g., storing image names for reading, loaders store calibrations and extrinsics separately in lists while GtsfmData stores a Dict of PineholeCameraCal3Bundler).

[travisdriver]

https://github.com/borglab/gtsfm/pull/258#discussion_r705441761