tests for elements, variometers, and observatories

@swilbur @erigler

These are very straightforward tests around checking the response formats for /elements/, /variometers/, and /observatories/ because these are all pretty static and don't change much.

I'm curious about some behavior I'm seeing and I'd love further insight if possible.

1. Why is "geometry" always set to None on elements? elements.py
2. Why do all of these resources get returned as an object with fields type and features instead of just as a list of resources? elements, observatories, variometers
3. Why the choice to transform the static data instead of returning the static data as it's stored in the files? Similarly, why nest most of the info under "metadata"?
4. Why aren't there separate classes for Observatories and Variometers?
5. Why are the /variometers/ endpoints methods called get_observatories or get_observatories_by_id?