Add optional name argument to add_constraint() method in ConstraintSatisfactionProblem() class

[JoelPasvolsky]

I can do:

csp1.add_constraint(operator.eq, ['a', 'b'])

I want to be able to do:

csp1.add_constraint(operator.eq, ['a', 'b'], name='aNOT_EQUALb')

so I can see:

csp1.constraints [Constraint.from_configurations(frozenset([(0, 0), (1, 1)]), ('a', 'b'), Vartype.BINARY, name='aNOT_EQUALb'),