how to warp existing annotations to a new template, using `brainglobe-registration`

[alessandrofelder]

Depends on brainglobe/brainglobe-napari-io#118

This should take the form of a "How-to" guide in the diataxis sense.

It should include the required prerequisites:

• a python environment with brainglobe-registration installed.
• the new template
• the desired annotations as a starting point for the new template

And the steps will include~

• finding the registration map between the old template (with the desired annotations) and the new template
• applying the registration map to the annotations
• exporting the annotations (to nifti if you want to use ITK snap for editing them)