add copy of method in builder

src/lib/src/main/java/gov/usgs/earthquake/nshmp/netcdf/data/NetcdfData.java

@@ -118,6 +118,16 @@ public class NetcdfData {
 
     Builder() {}
 
+    public static Builder copyOf(NetcdfData netcdfData) {
+      return builder()
+          .imts(netcdfData.imts)
+          .latitudes(netcdfData.latitudes)
+          .longitudes(netcdfData.longitudes)
+          .scienceBaseMetadata(netcdfData.scienceBaseMetadata)
+          .siteClasses(netcdfData.siteClasses)
+          .vs30Map(netcdfData.vs30Map);
+    }
+
     public Builder imts(List<Imt> imts) {
       this.imts = imts;
       return this;
@@ -162,5 +172,4 @@ public class NetcdfData {
       checkState(!vs30Map.isEmpty(), "Must add vs30s");
     }
   }
-
 }