abs in constraint_dc_current_*

[rb004f]

Add an argument to the constraint_dc_current_* family of functions (relaxation implementations) that allow the absolute value to be modeled with a relaxation or with an exact discrete variable.

Alternatively, create DDCP <: DCP, DWRMP <: DCP, DWRP <: DCP formulations, which imply that disjunctive variables are allowed to be used in the modeling of the formulation