handle inverted subclass queries

[cbizon]

Right now, if you have a query like this:
(a)-related_to->(b)-related_to->(c)
One valid response is:
(a)-subclass_of->(b)<-subclass_of-(c)

This is bad because b can be something like MONDO:0000001 or some other general node and you're basically saying that just about any A and C can be connected this way. Lots of (bad) answers.

There are a few approaches:

1. add a size filter on the degree of (b)
2. disallow subclass of edges in the body of the query.
3. Make use of the biolink hierarchy: there is a related_at_entity_level, so we could use that at query time instead of related_to. The problem with this is that it will require sub-predicates in the query which was one of the things that was causing slow/bad queries.