Automatic generator turnon/turnoff via operating year and retirement year

### Motivation

Some of the generators in our grid retire between 2016 and 2020, and some begin operating. It would be nice if we didn't have to manually keep track of which ones fall into which category, and to generalize this functionality to different years.

### Proposal

- Add two new columns to `plant.csv`, e.g. 'operating year' and 'retirement year'. Data would be ints or blank. @danlivengood has already compiled most/all of this data for the generators retired since 2016, and I could add this data for the generators added since 2016.
- Add a new method to the change_table, named e.g. 'set_simulation_year'. Given a year, this function would turn off (via change table scaling to 0) generators whose `operating_year` field (if it exists) is later than this year, and it would turn off generators whose `retirement_year` field is prior to this year.

### Urgency

This is a nice-to-have feature. We can keep on going with our current process through June, but once we have time it would be nice to get this in.