Interface between loaders and `GtsfmData`

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

```python
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_

The issue names only the loader interface and GtsfmData, with a proposed from_colmap_model class method; no files or tests are identified. Start by locating the existing COLMAP loader and GtsfmData definition, then determine whether the interface belongs on GtsfmData and verify the resulting loader behavior.

Written by the indexing model from the issue text.