Check brainmapper performance on single hemisphere datasets

[adamltyson]

See image.sc issue:
https://forum.image.sc/t/brainmapper-error/108718

This may end up being an issue with brainreg and how it saves deformation fields for hemisphere data.

[imagesc-bot]

This issue has been mentioned on Image.sc Forum. There might be relevant details there:

https://forum.image.sc/t/brainmapper-error/108718/8