Use half-step in, full step evolution

[mlxd]

Use the symmetry of the split-operator method to remove unnecessary transforms between evolution steps. Will require a re-working of the angular momentum section.

Perform half step into the algorithm, then perform the remaining evolution in full steps, until time to print out a wavefunction, when the half-step out is required.

Will remove some unneeded FFT's, multiplications and scalings per evolution step.