Support Methods to Query and Modify Disjunct Constraints

[pulsipher]

It would be useful to support methods that query information and modify disjunct constraints. These would include:

• The reformulation constraint (if appropriate)
• JuMP.set_start_value and JuMP.start_value
• JuMP.set_dual_start_value and JuMP.dual_start_value
• JuMP.set_normalized_coefficient, JuMP.set_normalized_coefficients, and JuMP.normalized_coefficient
• JuMP.set_normalized_rhs and JuMP.normalized_rhs
• JuMP.add_to_function_constant
• JuMP.value and JuMP.dual (if appropriate)
• JuMP.shadow_price (if appropriate)