def write_checksum(self):
return self._FITS.write_checksum(self._ext+<NUM_LIT:1>)<EOL>
Write the checksum into the header for this HDU.

Computes the checksum for the HDU, both the data portion alone (DATASUM
keyword) and the checksum complement for the entire HDU (CHECKSUM).

returns
-------
A dict with keys 'datasum' and 'hdusum'