Add relationship_type argument to GolrAssociationQuery

[deepakunni3]

Fix for monarch-initiative/biolink-api#201

Adding the flexibility of selecting a closure depending on a given relationship_type

[cmungall]

Looks good.

In future iterations of our golr schema, we should simply name the closure fields for entity and association docs according to the gp2term relationship. The current schema is based on
an early mistaken desire by me to "reuse" fields across golr-views.

In other words, isapartof_closure would be for GO ontology docs (since is_a and part_of are relations that hold e.g. between two processes), and involved_in and causally_upstream_of_or_within would be in assoc docs (since this holds between gp and a term) (cc @kltm and @balhoff )

This would obviate the need for a confusing mapping, where we say "for involved in use the isapartof closure" etc