first order non-symetric transfer. Calculates mass flow rate according to equation

mFlow = cA*CLa - cB*CLb

where cA and cB are concentrations at cport_a and cport_b. It might be either blood/plasma or tissue free concentration depending on the cBSwitch switch.

CLa - intercompartmental clearence for the A side

CLb - intercompartmental clearence for the B side