Consider subscripting change in electricity imports policy lever by power plant type

[robbieorvis]

WRI China asked us to consider subscripting this policy lever to control for which power plant contracts are being adjusted in imports. I think this is a relatively straightforward adjustment in the code. Practically it might make sense as well, as a lot of regions contract for power from specific plants and this would provide a bit more flexibility.

[jrissman]

Finished in commit 0ece5a9. This should make it into the 2.1 release.