available_expand: Return valid values for "expand"
Description
The GDC allows a shorthand for specifying groups
of fields to be returned by the metadata queries.
These can be specified in a select
method call to easily supply groups of fields.
Usage
available_expand(entity)
# S3 method for character
available_expand(entity)
# S3 method for GDCQuery
available_expand(entity)
Arguments
entity
Either a GDCQuery object
or a character(1) specifying a GDC entity ('cases', 'files',
'annotations', 'projects')