Add constants to model

[zirkel]

If we just use shared variables, they get added to the state. This isn't an efficient way to handle things. We should come up with another way to do this.

[Stephen Siegel]

After talking with Tim, we decided (a) there is no difference between a constant and an input, and (b) neither one is actually constant, because of simplification (e.g., "X" might get replaced by "10" or "Y" or anything else). Hence they need to stay in the state. So no change is necessary.