RTree API improvements and more tests

This change reverts the RTree API back to the original one (i.e.
insert() instead of put(), search() instead of find()), and
creates a new searchReturningObject() method that returns an
object keyed by UIDs.

It also adds missing tests for type restricted search and
searchReturningObject().
Merge pull request #731 from ahocevar/rtree-api-tests

RTree API improvements and more tests. r=@twpayne