Fix coating brownian so it agrees with Hong

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.

Source of the plot

gwinc/noise/coatingthermal.py

@@ -91,7 +91,7 @@ def coating_brownian(f, materials, wavelength, wBeam, dOpt):
 
     # layer contributions, b_j (eq 1) from doi:10.1103/PhysRevD.91.042002, errors corrected
     brLayer = ( 1/(1-pratN) *
-                ( (1-nN*dcdp_z/2)**2 * (1-2*pratN)*(1+pratN)*Ysub / ((1-2*pratsub)*(1+pratsub)*yN) +
+                ( (1-nN*dcdp_z)**2 * (1-2*pratN)*(1+pratN)*Ysub / ((1-2*pratsub)*(1+pratsub)*yN) +
                   (1-2*pratsub)*(1+pratsub)*yN / ((1+pratN)*Ysub) ) )
 
     # sum them up for total