* Use HTTPStatus for all responses
More informative than just the code as an int
* Fill out REST error handling
* Test error routes
* Use HTTP Status for tests too
* factor mime type request into function
* Better mimetype errors
* 404 -> 400 error for bad key
REST API now requires that the server sort all dataframe responses by dimension index (var or obs index), allowing the client (front-end) to assume responses will be sorted. Given that the dataframe usually has a natural ordering, and the server gets to arbitrarily assign indices, this is the most computationally efficient design.
This PR includes changes to the REST API spec, and removing the explicit sort in the front-end code.
