Ability to pass agent parameters to constraints

There is currently no way to pass agent parameters to the constraints. This isn't a problem for the constraints that are currently used, as there aren't any agent parameters that they would use, but I imagine this could be useful for some hypothetical future constraints.

For example, we discussed adding a constraint that puts a cap on total fuel costs. Obviously this cap would need to be a user-configurable parameter, so we would need some way of getting this parameter to the constraint.

Shouldn't be too difficult, but just something to bear in mind