Allow early decommissioning

Currently, once an asset is installed, it can never be decommissioned until it reaches its technical life. Commodity demands in a given year will first be met by existing assets operating at full capacity, and then investments are decided on to meet any leftover demand.

Often, allowing assets to run for their full lifespan is the most sensible cost-minimisation strategy. However, in some cases it may actually be cheaper to decommission assets early and install new assets rather than paying high running costs for existing assets. An example would be if there's a substantial increase in the carbon price that makes high-carbon technologies no longer cost-effective to run.

I imagine this would be require substantial changes to the code, but something we could take into consideration if/when reworking the scipy solver (as discussed in #389)