implementation of Spudich et al. (1999) for PRVI 2003

Implement Spudich et al. (1999) with the intention to replicate fortran code used for PRVI 2003 calculations. Since we don't have the original source code for that model, some implementation details have been taken from and compared to fortran code hazgridXnga.samoa.f.

See #266 (closed)

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

@@ -1563,6 +1563,13 @@ public enum Gmm {
       UsgsPrviBackbone2025.COEFFS_SIGMA_SUBDUCTION,
       UsgsPrviBackbone2025.SlabTotal.CONSTRAINTS),
 
+  /* PRVI 2003 */
+  SEA99_PRVI(
+      SpudichEtAl_1999_PRVI.class,
+      SpudichEtAl_1999_PRVI.NAME,
+      SpudichEtAl_1999_PRVI.COEFFS,
+      SpudichEtAl_1999_PRVI.CONSTRAINTS),
+
   /* Combined: must be declared after any dependent models above. */
 
   /** 2021 Hawaii weight-averaged GMM for deep earthquakes. */