[Feature] Just one Kim dev top-level mesh

[alessandrofelder]

Is your feature request related to a problem? Please describe.
The Kim developmental atlases currently have two root meshes ("root" and "mouse" - "mouse" is a child of "root", and the parent of everything else).

Describe the solution you'd like
Probably not create our own root mesh in the packaging script, and store the mouse mesh (which presumably is provided by the non-BrainGlobe side) as the root mesh?

Describe alternatives you've considered
Leave as is, maybe? Would be less consistent with other atlases though (but more backwards compatible for existing users and user code)

Additional context

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