Add .get_entities() method to BIDSLayout

Currently, you can get valid entities by running BIDSLayout(bids_root).entities and a little parsing. It would be nice to have a helper function such as bids.layout.list_entities():

def list_entities(domains=('bids',)):
    res = []
    for domain in domains:
        res.extend(...)  # Parse the entities from the JSON files
    return res

The reason this comes up is that, currently, nipype.interfaces.io.BIDSDataGrabber manually parses the JSON files, found in a path relative to bids.__file__. This isn’t great.

[tyarkoni]

Following up on this, in 0.8, you can do Config.load('bids') to get back a Config object that has a dictionary of entities only valid for that config. This doesn’t require populating a layout, but it only works for a single config at a time. So at this point it would be very straightforward to add a static method in BIDSLayout that takes a list of config names and returns the union of available entities across all resulting Config objects.

[tyarkoni]

0.9 adds a get_entities() method that accepts scope and is_metadata arguments, where the former controls where to look for entities, and the latter allows you to restrict to only metadata or non-metadata entities if desired.