future pgv refactor comment

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

@@ -138,6 +138,7 @@ class UsgsPgvSupport {
    * @return ScalarGroundMotion for target T(PGV)
    */
   static GroundMotion calcSaGroundMotion(Gmm gmm, GmmInput in) {
+    // TODO use list
     Set<Imt> saImts = gmm.responseSpectrumImts();
     // need to verify that this Set is being returned with ascending
     // iteration order; that is, when we loop over them