Refactor how process maps are represented and used

This will allow new features (e.g. #845), but is also necessary to fix some bugs (e.g. #846).

I think ultimately we should store the individual entries in the CSV files in the `Process` struct, deduplicating by using `RangeMap`s (see #874). We will need to transform this data into something usable when we construct an `Asset`. We should ensure entries are provided for every milestone year upfront, but for other years we should do the check lazily to allow for assets whose commission year is on a non-milestone year (#859).