add compare to

src/lib/src/main/java/gov/usgs/earthquake/nshmp/netcdf/Netcdf.java

@@ -18,7 +18,7 @@ import gov.usgs.earthquake.nshmp.netcdf.reader.Reader;
  *
  * @author U.S. Geological Survey
  */
-public abstract class Netcdf<T> {
+public abstract class Netcdf<T> implements Comparable<Netcdf<T>> {
 
   protected final Path netcdfPath;
   protected final NetcdfData netcdfData;
@@ -84,5 +84,11 @@ public abstract class Netcdf<T> {
    */
   public abstract T staticData(Location site, NehrpSiteClass siteClass);
 
+  @Override
+  public int compareTo(Netcdf<T> that) {
+    return this.netcdfData().scienceBaseMetadata().label
+        .compareTo(that.netcdfData().scienceBaseMetadata().label);
+  }
+
   abstract Reader getNetcdfData(Path netcdfPath);
 }