    util.normalizeSides(box)

Return a new object of the form `{ top: Number, bottom: Number, left: Number, right: Number }`. The value for each of the side is determined by the argument `box`. If `box` is a number, all for sides will have this number. If it is an object with some/all of the `right`, `left`, `top`, `bottom` properties, these properties will be used. If any of the properties is missing, they will be set to `0` for the related side in the resulting object.

    joint.util.normalizeSides(5) // { top: 5, left: 5, right: 5, bottom: 5 }
    joint.util.normalizeSides({ left: 5 }) // { top: 0, left: 5, right: 0, bottom: 0 }

JointJS and Rappid use this method internally whenever there is an option object that can be specified either by a number or a (possibly incomplete) object with sides (for example, the `padding` option).