Appraisal of divided assets is inefficient

[alexdewar]

The way in which divided assets (child assets) are appraised currently is inefficient. Since #1092, we only consider one child per round of appraisal, but that can still mean we end up with many rounds of appraisals. It would be much more efficient if we could decide in one pass how much of the aggregate capacity of the children we want (as represented in the parent asset) and then decommissioning any that are surplus to requirements.

We allow the capacity variable to vary within limits for new assets, in order to determine what its ideal capacity should be, so I imagine we should be able to do something similar for child assets. Perhaps the best way would be to get the parent asset for any child assets, perform appraisal with the parent, then decommission the appropriate number of children depending on the result of the appraisal.

While working on this, bear in mind we have a longer-term goal of removing child assets altogether (#1123), which will ultimately make this code cleaner.