# Step.m

Propagation step function. Uses Krylov propagation and sparse exponenti- ation where appropriate. Syntax:

                  rho=step(spin_system,L,rho,time_step)


Arguments:

     L          -  Liouvillian or Hamiltonian to be used for propagation

     rho        -  state vector or density matrix to be propagated

     time_step  -  length of the time step to take