This will give us the ability to specify different config options for
different dataroots.
the key of the dataroot dictionary is no longer the same as the dataroot_url.
Previously key==dataroot_url, and now those are separated.
Added an "is_multi_dataset" function to simplify logic where it branched on single vs multi.
Simplified the rest.py interface by no longer passing in the user annotations object, since
that can be retrieved from the dataset.
f69d141336