Group graph: when exporting the adjacency matrix, names are already managed,
considering both groups and split neurons.
Circuit graph plot: getSkeletonModels must return an object, not an array.
Circuit graph plot: 'Refresh' button must use getSkeletonModels.
Circuit graph plot: rename variable for clarity.
Circuit graph plot: fix error when refreshing, which involves re-appending all.
Circuit graph plot: fix error when re-plotting, visible e.g. when resizing
the widget when there's only one skeleton added and therefore no plot present
and no vectors computed yet.