Decide on Fermi energy for DOS and Band

[martin-schlipf]

For metals it should be more transparent how the Fermi energy in VASP is selected. The issue is that when the band structure is plotted a new Fermi energy is computed but it may be less accurate then the original one because the mesh is coarser.

Possible solutions: Add an optional argument to the plot routine that sets the Fermi energy manually. The documentation needs to be very clear so that it is transparent what the default shift is.