Add a single range constraint to a model. A range constraint
states that the value of the input expression must be between
upper bounds in any solution.
Note that range constraints are stored internally as equality
constraints. We add an extra variable to the model to capture the
range information. Thus, the
attribute on a range constraint will always be
expr: Linear expression for new range constraint.
lower: Lower bound for linear expression.
upper: Upper bound for linear expression.
name: Name for new constraint.
New constraint object.