Add support for easily adding magnetic field to a system.
There are several tricky aspects of the problem:
- Magnetic field doesn't always enter as an overall phase factor. In superconducting systems, the Peierls phases of electrons and holes are opposite.
- Optimally, the user shouldn't need to fix the gauge on her own, and a gauge automatically compatible with the leads should be selected.
- We need to decide what happens if there are next-nearest neighbor hoppings, are hoppings always assumed to lie on a "straight line"?