Re-implement dynamic generation portfolios

• aka Multi-year simulations with discounted costs
• aka Power (and other) plant sets that can change over the simulation (decommission old units, build new units in different years)

There is an old branch Dynamic_generation_portfolios for adding functionality for multi-year horizons, discounted costs, rolling horizon in the capacity expansion planning, and start and end years/samples/timesteps of units. The branch was never merged to dev branch but similar changes could be re-implemented and merged to dev.

One big question is whether it should be possible to invest in the same unit/link more than once (i.e. at different t_invest), so that the capacity of that unit/link would gradually grow (and decrease when reaching the end of its lifetime). At the same time, some of its parameters, like the investment cost, could also change, but not all. This would bring some savings in the number of variables/equations but the code would become more complicated. Another option would be to force that a new unit has to be created for each new investment (made at different t_invest), even if the technology stays the same.