Add `mothball_years` model parameter

[alexdewar]

In the document it is mentioned in passing that assets should be decommissioned if they are unused for MothballYears.

Presumably the model will work without this feature, so it's probably more of a "nice to have". I'll ask Adam about it though, before we kick the can down the road on this.

[alexdewar]

Adam has suggested this issue for this engagement (see #911).

To do this, we will need to track for how many years each process has had no activity and then automatically decommission any that exceeds the limit specified by mothball_years.

I think the mothball_years parameter should default to no limit.