Gets a specified value of a dual variable.
     
    
    extract_dual_value(result_vec, offset, constraint)
 
     
    
    Arguments
- result_vec
 
A vector containing the dual variable values.
 
- offset
 
An offset to get correct index of dual values.
 
- constraint
 
A list of the constraints in the problem.
 
 
    
    Value
    A list of a dual variable value and its offset.