Specifies the section (and so all the segments associated with it) to be in a particular group
Note: only one of SOMA_GROUP, DENDRITIC_GROUP, AXONAL_GROUP is allowed in the set of groups
Note: the delay associated with the Action Potential propagating between rpre & post
connection points could in theory be calculated each time from cell positions and morphology,
but it's more efficient to cache it here when the connections are generated