Compute centroids

[gkiar]

When generating a graph, compute centroids of the rois in the given atlas, and add as a vertex attribute

[jovo]

@loftusa this is a not a "graph specific" thing, rather, it is an "atlas specific" thing.
in any case, for each atlas, we should record the centroids.
a bunch of people are now asking us for that.

@ebridge2 is working on it.
@bdpedigo can help, since he wants these things too.

[j1c]

@jovo Seems like this is something that should be added to the jsons im working on?

[jovo]

@j1c
agreed!