Remove child assets and use parent assets everywhere instead

[alexdewar]

Now that we have added parent assets to divided assets, we should be able to use that parent asset everywhere in the code instead of evaluating each child separately (remember they are identical).

There are a number of things we have to consider before we can do this though:

• How will we handle the case where there is a partially decommissioned divided asset? Currently it's easy -- each of the "parts" is a separate asset -- but when it is one object representing all parts, this will be more tricky. Do we need to represent the decommissioned part as a separate object in the decommissioned pool?
• Formats for output files (Reconsider output format in the context of divisible assets #1042). Do we want to keep output files the same? For many child assets, they could end up being very verbose, but it's not obvious how they could be combined in every case (e.g. for the assets.csv file, different children will have different decommission_years). If we do keep them the same, this will mean that parent assets have to track all the different IDs representing their children.
• We need to figure out how to appraise parent assets (Appraisal is being run on all assets in a group #1045) and partially decommission them without using child assets