Return the list of timers associated to the given `filename` and MPI rank mpi_rank.

def timers(self, filename=None, mpi_rank="0"):
        """
        Return the list of timers associated to the given `filename` and MPI rank mpi_rank.
        """
        if filename is not None:
            return [self._timers[filename][mpi_rank]]
        else:
            return [self._timers[filename][mpi_rank] for filename in self._filenames]