getPortExtModel

      getPortExtModel   Returns error term for the port-extension model.
                        Port-extension is generally measured with a reflective
                        termination at the desired measurement plane. This can
                        be either an open-circuit or an actual open-standard
                        termination.
          e10e01 = getPortExtModel(gamma, gamma_listed)
          input:
              gamma,gamma_listed: [num_pts] measured and listed reflection
                  coefficient at the extended port
          output:
              e10e01: [num_pts] Transmission tracking coefficients of the error model