Decide which parts of the RMA spec to implement.

This is a follow up to https://github.com/BlueBrain/Atlas-Download-Tools/pull/42#discussion_r689450383

There are nearly infinite possibilities for abstractions of the AIBS RMA grammar. It might be beneficial to include some of them as part of our library, but we need to think about what exactly to include/abstract.

References:
- [RMA Builder](http://api.brain-map.org/examples/rma_builder/rma_builder.html)
- [RMA Syntax](http://help.brain-map.org/display/api/RESTful+Model+Access+(RMA))
- [Class Hierarchy](http://api.brain-map.org/class_hierarchy)