Investigate 3D expression grid (part of the Allen API)

[jankrepl]

Might be actually really useful (potentially simplifying our pipelines).

http://help.brain-map.org/display/api/Downloading+3-D+Expression+Grid+Data

TODO:

• Can one choose the resolution or is it fixed to 200 microns?
• How to read it in Python?
• Is the volume "synchronized"?
• Can we reproduce these volumes with our current pipelines?
• What kind of processing do they apply to these volumes?