PROV-684 Added additional options to unit tag

We now support two additional options:

"restrictToTypes" and "restrictToRelationshipTypes"

They restrict the context switch by type of the record or by
relationship type. Both are passed to the unit tag as attribute.
Multiple values are delimited by "|", for example:

<unit relativeTo="ca_entities" restrictToTypes="person|institution"
restrictToRelationshipTypes="artist" delimiter=";
">^ca_entities.preferred_labels</unit>