added depth scaling method

src/main/java/gov/usgs/earthquake/nshmp/gmm/ChapmanGuo_2021.java

@@ -34,6 +34,7 @@ class ChapmanGuo_2021 {
       Imt.mprsImts().toArray(new Imt[0]));
 
   static final double VS_REF = 1000.0;
+  static final double Z_CUT = 0.5;
 
   private static final double[] Z = {
       0.0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8,
@@ -78,6 +79,16 @@ class ChapmanGuo_2021 {
     return data;
   }
 
+  /**
+   * Return the depth scaling factor for application of reference site scaling.
+   * Curently set to 0.0 at z = 0.0 km and 1.0 at z = 0.5 km.
+   *
+   * @param z sediment depth (in km)
+   */
+  static double zSiteScale(double z) {
+    return Math.min(1.0, z / Z_CUT);
+  }
+
   /**
    * Return the psa ratio for the supplied sediment depth, magnitude and
    * distance.