redesign: mesh.get(scalar) returns single mesh instead of dict

[william-silversmith]

It's pretty annoying to write:

m = cv.mesh.get(23948324)[23948324] 

It would be more idiomatic to write:

m = cv.mesh.get(23948324)

This would break the existing interface though. Would everyone be okay with that?

[manuel-castro]

It would be nice to keep the original interface in the case of passing a list of segids with length 1. Besides that, sounds good to me

[william-silversmith]

That's the idea. I'm just worried about breaking people's scripts now that the feature has been this way so long. :-/