[Feature] minor improvements to axolotl atlas

[alessandrofelder]

Is your feature request related to a problem? Please describe.
As we wrap up the axolotl atlas packaging, this is a place to take note of possible improvements that we could do in the future

• mask the reference image with the annotations, to allow this to be a registration target (and keep the original reference as an additional reference)
• improve the meshes (some of them seem to have meshed some spurious pixels, some only exist for one hemisphere?)
• add the 80+ ROI annotations (there are two annotations, and we've only packaged one)
• improve the abbreviations (remove the numbers?)

Describe the solution you'd like

Turn the above bullet points into issues and address them