Add `MaxImbalance`, `MinImbalance`, `TargetImbalance` constraints

[enzbus]

Connected to #180. It's often unclear to users how the cash weight is connected to the longs and shorts. Create 3 extra constraints to specify the long/short legs imbalance they require. NoCash and DollarNeutral become special cases of TargetImbalance. Documentation will explain how they are used.