Base class for the transport properties of a Helmholtz fluid.
The default implementation does NOT return a value: mu() and lambda() raise BELFEM_ERROR, so asking a fluid without a transport correlation for one aborts the run rather than handing back a silently wrong number. A species with a real correlation derives from this class: HelmholtzTransport_Methane, HelmholtzTransport_Hydrogen and HelmholtzTransport_LemmonJacobsen ( nitrogen, oxygen ).
Every Helmholtz fluid Gas can build currently carries one of them; the base is only reached by a fluid added without a correlation.