feat: observations/place_count endpoint

[kfc35]

A continuation of #511 to address kueda’s comments, adds a new parameter called count_place_id to filter in the desired places. I found a nifty “include” attribute to do this. I also added a custom order_by so you can only order by id or count.

I wrote a couple more tests to test out the attribute for the endpoint (which, if my earlier pull concerning new test methods is merged first #542 and you want to wait a little bit, I can use those methods instead).

Closes #56
Builds upon & Closes #511

If a v2 endpoint is desired, please let me know; I can add one.