Return as tree map

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

@@ -5,6 +5,7 @@ import static com.google.common.base.Preconditions.checkState;
 import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
+import java.util.TreeMap;
 
 import gov.usgs.earthquake.nshmp.geo.Bounds;
 import gov.usgs.earthquake.nshmp.geo.Location;
@@ -90,7 +91,7 @@ public class NetcdfData {
    * Returns the VS30 map
    */
   public Map<NehrpSiteClass, Double> vs30Map() {
-    return Map.copyOf(vs30Map);
+    return new TreeMap<>(vs30Map);
   }
 
   public static Builder builder() {