Priority system in loaders no longer works

[nmearl]

Loaders are assigned priorities and they should be taken into consideration when conflicts arise amongst potential loaders when a format is not explicitly give.

[dhomeier]

Those are passed as kwarg to data_loader, right? But where are they (supposed to be) assigned? I'd expect they might be written into the @data_loader decorators, but can't find one that has a priority value or seems to have had one in recent times...

[nmearl]

The loader priorty is stored here, but it looks like we need to be more specific about using it (before, it seemed that the order in the registry was part of the determination for which loader to choose). It seems at some point, we took them out during a refactor.

I'm imagining that we'll want to find all loaders that meet the identifier criteria. If there is more than one, simply compare priorities and return the appropriate loader.

[aragilar]

This is fixed by #757.