[MRG] ENH: Small improvements for agents.py (#1139)

* ENH: Small improvements for agents.py

* FIXUP: fix `add_thing` to pass the tests

* [MRG] ENH: Add small chnages to agents.py

* [MRG] FIX: `default_location` now returns a valid location

* FIXUP: fix `default_location` in agents4e.py and modify tests