Improve printing

[jd-lara]

The current printing of a constraint with params can be confusing since it adds a 0.0 on the RHS.

julia> m = ModelWithParams();
julia> a = Parameter(m, 10);
julia> x = @variable(m);
julia> t = @constraint(m, x <= a)
noname ≤ 0.0

[joaquimg]

Uhm, thats because the model is only synced once all variables are added.
Maybe we can add a mode to passa the constants directly.
Having parameters themselves in the printing might be harder.

[jd-lara]

Agreed, this seems to be a hard problem but it's worth registering it into the issues for reference.

[joaquimg]

One option would be to create a ParametricConstraintRef type.