od_aggregate: make sure aggzones are not intersecting

[idshklein]

when using od_aggregate, st_join sometimes joins 2 polygons to an o or d, due to the fact that aggzones are overlapping, and the o or d resides in both.
the function should throw an error when aggzones are intersecting.

I'll supply a reprex if requested

[Robinlovelace]

Hi @idshklein great to see od is being used, by someone other than me and colleagues in ITSLeeds! Please do supply a reprex, could be a solid foundation for a fix to this issue.