Initial implementation of brain-atlas endpoints (#195)

See #168 for the specification.
Briefly; a `BrainAtlas` is an named entity that has an asset `annotation.nrrd` attached to it.
It is associated with a particular `BrainRegionHierarchy`
In addition, `BrainAtlasRegion` gives metadata for all the regions within a `BrainAtlas`;
* their volume in the `annotation.nrrd` if they are a leaf, None otherwise.
* an attached asset with the .obj mesh

Author: mgeplf