Deduplicate process maps by year

Currently we create process maps (availabilities, flows and parameters) for every possible year a process can be commissioned in upfront, which will be a problem when we get to #845 and #846, as at that point the number of possible years for a process could be thousands or even billions (assuming we treat missing lower and upper limits as being 0 and `u32` max, respectively).

We could work around this partially, by capping the range of possible years, but it would be better to just not create many copies of these maps in the first place.

I think we should use the [`rangemap`](https://crates.io/crates/rangemap) crate so we can store values for ranges of years.