Krylov
 subspace approximation of the action of the exponential
 (see M. Hochbruck and C. Lubich, SIAM J. Numer. Anal. <b>34</b>, 1911 (1997) for details).
 Two more variables control the performance of the method: the maximum dimension
 of this subspace (controlled by variable <tt>TDExpOrder</tt>), and
 the stopping criterion (controlled by variable <tt>TDLanczosTol</tt>).
 The smaller the stopping criterion, the more precisely the exponential
 is calculated, but also the larger the dimension of the Arnold