Phs/relation get and scan can read whole relations

`Relation#create_get` get family options, and `families: :relation`.  `families: :relation` includes all families in the relation schema.  Scans get this new option as well.

`ExecutionContext#relation` considers all families when passed an op. This way, if you get a relation back at all, you can be sure all families (say, in a passed put) are within.