Remove conversion of `GtsfmData` Dictionaries to Lists

I spent a while trying to update the code so that we do not rely on converting the dictionary of cameras in `GtsfmData` to a List. However, almost all of the metric computation functions rely on Lists of Pose derived from the camera dictionary of GtsfmData.

I'm assuming this is why the `_number_images` attribute of the `GtsfmData` class was added in the first place: so that the cameras data could be converted to a list such that its the same length as the original ground truth data. I think this is should be saved for another PR.

I do think that using Dicts instead of Lists like Colmap is a lot better and should be implemented in the near future.

_Originally posted by @travisdriver in https://github.com/borglab/gtsfm/pull/376#discussion_r750876123_

Start by locating the GtsfmData camera dictionary and the metric computation functions that currently derive Lists of Pose from it. Confirm how _number_images and ground-truth length assumptions are used; done should mean the dictionary representation no longer needs conversion to Lists without breaking those computations.

Written by the indexing model from the issue text.