Translate Query(Enum) into ancp-bids terms

[effigies]

In PyBIDS we have the idea of layout.get(desc=Query.REQUIRED) (where desc is an arbitrarily selected entity) that indicates that any non-null value is permitted. Query.OPTIONAL permits the value to be present or absent. Query.NONE only matches files do not have that entity.

These queries should be translated to ANCP equivalents.

[erdalkaraca]

This is already (partially) implemented in the query language expressions, but need to be reflected in the BIDSLayout.get() docs:

        filters : dict
            Any optional key/values to filter the entities on.
            Keys are entity names, values are regexes to filter on. For
            example, passing filters={'subject': 'sub-[12]'} would return
            only files that match the first two subjects. In addition to
            ordinary data types, the following values are defined:
                * None: The named entity must not be defined.
                * '*': the named entity must be defined, but can have any value.

What is missing is Query.REQUIRED, though there might be semantic overlap with Query.OPTIONAL:

layout.get(desc=Query.REQUIRED) == layout.get(desc=Query.OPTIONAL)

translates to:

layout.get(desc='*')

=> value can be anything, i.e. optional but not None