Consider adding the ability to specify one axis bound to `BinaryVariable`

[arcondello]

Very often we need constraints such as

model = Model()

x = model.binary((5, 5))

model.add_constraint((x.sum(axis=0) == 1).all())

which specifies that each column can have exactly one 1 in it.

It would be nice to be able to make such constraints implicit. Maybe an API like

x = model.binary((5, 5), subject_to=(axis, operator, rhs))

so the above example would be

x = model.binary((5, 5), subject_to=(0, '==', 1))

This would also allow us to deprecate the DisjointBitSets and DisjointBitSet symbols.

[fastbodin]

Are you envisioning having attributes for BinaryNode like

std::vector<double> axis_lower_bounds_;
std::vector<double> axis_upper_bound_;

to store / interpret the subject_to(axis, operator, rhs)?