BQS — query-able and non query-able attributes

Repository introspection mechanism allows to find what attributes are
available in the repository. But the attributes play two roles: they can
be used in queries (query-able attributes) and they can be just return
back in the retrieved citations (usually the first role is a subset of the
latter one). There is no interoperable way how to find what role which
attribute plays.

Resolution:

To add to DsLSRBibQuery module:
const string ROLE_ATTR_QUERYABLE = "queryable";
const string ROLE_ATTR_RETRIEVABLE = "retrievable";

And to explain in the text that these strings could/should appear in the
'description' field of the controlled vocabularies with attributes.

see above