eliminate min up/down time constraints for units with min up/down = 1

[rdzman]

From https://www.mail-archive.com/matpower-l@cornell.edu/msg06940.html ...

It looks like this would be a good place for some optimization of the code. I believe it’s true that the min up and down time constraints are essentially unnecessary when the min up and down time values are 1. It seems to me that the only difference is that it becomes feasible to have a startup and shutdown event (v = w = 1) for a given unit at a given period, resulting in no change in state. However, this should have no impact unless the startup and shutdown costs are non-zero, in which case that solution would not be optimal. So, I believe, simply eliminating these constraints when the min up/down times are 1 should result in an equivalent optimization problem.

To optimize this code to handle the general case more efficiently for large systems, it seems like maybe we should be indexing these constraints by (t, tau) instead of (t, i).

[rdzman]

Building of min up/down time constraints was optimized significantly in 7923b99 (single constraint set per period, not per period per generator). So the setup time should no longer be the bottleneck it was previously.

However, the unnecessary min up/down time constraints for units with min up/down time of 1 have not yet been eliminated, so they may still affect solve times.