Represent groups of related reservations
As mentioned in https://gitlab.flux.utah.edu/powderrenew/powder/issues/21 we need to be able to represent groups of reservations that are logically treated as one, though they don't need to be implemented this way down in the reservation system itself. Some additional thoughts on this can be found in https://gitlab.flux.utah.edu/powderrenew/powder/issues/86 .