return the samtools usage information for this command

def usage(self):
        '''return the samtools usage information for this command'''
        retval, stderr, stdout = _pysam_dispatch(
            self.collection,
            self.dispatch,
            is_usage=True,
            catch_stdout=True)
        # some tools write usage to stderr, such as mpileup
        if stderr:
            return stderr
        else:
            return stdout