Remove conversion of GtsfmData Dictionaries to Lists

[travisdriver]

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 #376 (comment)