disagg interface system fix

src/main/java/gov/usgs/earthquake/nshmp/calc/DisaggDataset.java

@@ -491,6 +491,9 @@ final class DisaggDataset {
           case FAULT_SYSTEM:
             putOrAddSystem((SystemContributor) disaggContributor);
             break;
+          case INTERFACE_SYSTEM:
+            putOrAddSystem((SystemContributor) disaggContributor);
+            break;
           default:
             putOrAddSource((SourceContributor) disaggContributor);
         }

src/main/java/gov/usgs/earthquake/nshmp/calc/Disaggregator.java

@@ -91,6 +91,8 @@ final class Disaggregator {
         return processClusterSources();
       case FAULT_SYSTEM:
         return processSystemSources();
+      case INTERFACE_SYSTEM:
+        return processSystemSources();
       default:
         return processSources();
     }