Add GET parameter for specifying the query processor

Fix #188.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
This took me a second to understand what was going on. For this resource, using a modeltree doesn't really make sense since the context *is* the field we are returning stats about. So the instance model *is* the modeltree. You can pass the model as the `tree` argument and it makes this `if` block moot.