[FEA] Add support for semi-continuous variables

[chris-maes]

A semicontinous variable satisfies X = 0 or L <= X <= U with 0 < L < U

• If U < Inf, we can model as LB <= X <= UB with additional binary variable B
• If U = Inf, use bound strengthening to try to get finite upper bound on U
• If U still Inf, set U=big-M, allow user to set big-M via parameter