Fix an error in the coating brownian noise calculation

A discrepancy was found by Gabriele Vajente between the gwinc CTN calculation and several other codes, especially in the case of large Poisson ratio in the coating.

See: https://git.ligo.org/gabriele-vajente/thermal-noise-models

This commit resolves the discrepancy by fixing an apparent error in the Yam et al calculation that was implemented in gwinc.

gwinc/ifo/Aplus/ifo.yaml

@@ -212,7 +212,11 @@ Materials:
     ThermalDiffusivitylown: 1.38  # Fejer et al
     Indexlown: 1.45
     Philown: 1.25e-5              # silica mechanical loss
-    Philown_slope: 0.4
+    Philown_slope: 0              # G1600641 and arXiv:1712.05701 suggest
+                                  # slopes between 0 and 0.3, depending on
+                                  # deposition method. Slawek's analysis in
+                                  # 10.1103/PhysRevD.98.122001 assumes zero slope.
+
 
   ## Substrate Material parameters--------------------------------------------
   Substrate: