Unpractical type casting for Vector Constraints

[PierreTsr]

Currently all the constraints that take a vector of variables as input are casted as Vector{AbstractIntVar}. This is useful in order to define constraints on IntVar but also on IntVarView.
However, by doing it that way, the user is forced to cast its vector as AbstractIntVar eventhough the vector contains only a more specific variable type, which is quite unpractical.

This could be fixed easily by casting the input of these constraints as Vector{<:AbstractIntVar}, and it would even allow a more specific dispatch in the constraints if needed.

[qcappart]

I agree , it seems to be a suited improvement. @ilancoulon and @felixchalumeau do you see any issue with this ?

[ilancoulon]

I think it is a a great idea!