Cfmagnus

First implementation of the Commutator-Free Magnus propagator, as described in section 3.2 of https://arxiv.org/abs/1803.02113. As it is right now, it only serves to propagate systems with non-moving nuclei.

Besides the obvious implementation of the propagator and its interface in the '/td' section of the code, a modification in the 'hamiltonian.F90' file was needed to update the Hamiltonian with a weighted sum of the external potentials at two different times.