Add logger to N3headerNew that CDF5 is not supported (#1350)

Author: rschmunk

cdm/core/src/main/java/ucar/nc2/internal/iosp/netcdf3/N3headerNew.java

@@ -38,6 +38,9 @@ public static boolean isValidFile(ucar.unidata.io.RandomAccessFile raf) throws I
       case NETCDF3:
       case NETCDF3_64BIT_OFFSET:
         return true;
+      case NETCDF3_64BIT_DATA:
+        log.debug("NETCDF3_64BIT_DATA (CDF5) not supported");
+        return false;
       default:
         return false;
     }