Express "exactly equal" constraints

[bmaclach]

Currently in Drasil, constraints can either be expressed as a Range, EnumeratedReal, or EnumeratedStr.

Another type of constraint we need to express are constraints that one thing is exactly equal to another. For example, in SSP we have the constraint that the size of the lists x_slope and y_slope are equal (and similar for other lists that represent x and y coordinates for different parts of the system).

This could potentially be expressed as a Range where the upper and lower bounds are the same, though that seems like a hack.

[JacquesCarette]

In theory, that could be done with dependent types (and should be). Except that Drasil isn't likely to get such a type system anytime soon. In the meantime, extending the kinds of usable constraints to contain an Equality case makes the most sense.