Group divided assets under a parent asset #1100
Description
As discussed with @tsmbland, I think we can probably refactor things so that divided assets have "parents", which can be used in contexts where we don't want to perform the same calculation on child assets over and over.
The parents' capacity will need to be updated as children are decommissioned, so that it continues to represent the total capacity.
See discussion here: #1092 (comment)