The small remaining discrepancy between Hong and gwinc is due to
an error in gwinc's implementation of the Yam et al model.

The coefficient in front of the phase derivative `dcdp_z` is
supposed to be `nN` (as written correctly in Yam et al, eq 1),
not `nN/2` (as found in gwinc).

In Hong et al, the coefficient is `nN/2`, but that's because
their phases are defined as one-way instead of round-trip (see
Yam et al, eq A2).

After correcting the error, gwinc and Hong agree, as expected,
for the case of equal bulk and shear losses and no
photoelasticity.