cdm/src/main/java/ucar/nc2/NetcdfFile.java
static public NetcdfFile open(String location, ucar.nc2.util.CancelTask cancelTask) throws IOException {
    return open(location, -1, cancelTask);
  }
static public NetcdfFile open(String location, ucar.nc2.util.CancelTask cancelTask) throws IOException {
    return open(location, -1, cancelTask);
  }
Open an existing file (read only), with option of cancelling.

@param location   location of the file.
@param cancelTask allow task to be cancelled; may be null.
@return NetcdfFile object, or null if cant find IOServiceProver
@throws IOException if error