Metadataset improvements

[BrookRoberts]

Allows masks to be used in metadatasets, and uses absolute paths as opposed to relative paths for specifying location of datasets.

Previously running:
bin/opensfm create_submodels data/my_dataset
Would not copy across masks
Would have broken symlinks for all of the images.

Note:
How are you expected to use metadataset? Is it a useable feature? It doesn't seem to be documented anywhere. If you create the metadataset without previously finding features and matches (or at least creating the folders) it creates broken symlinks and doesn't work when you try and run opensfm_run_all on the entire set.
However, if you do run detect_features and create_matches then when you run opensfm_run_all on the subsets it tries to rematch. This would be fixed with #178.

I think it would be good if there was a similar executable to opensfm_run_all that ran with metadatasets. Happy to make this with one of the above flows, and happy to make fixes so both the above flows works (assuming it's just minor fiddling), but would depend if there is currently an expected way of doing things.

I've not looked, but I presume align_submodels requires matches across submodels, and thus the correct thing is for feature detection and matches to be done on the entire set.