Allow shifting bands plot with different Fermi energies

[qiaojunfeng]

Useful for comparing two band structures computed with different pseudopotentials (thus the Fermi energies can be different). For instance,

from aiida_wannier90_workflows.utils.workflows.plot.bands import get_mpl_code_for_workchains

mpl_code = get_mpl_code_for_workchains(workchain_sssp, workchain_dojo, shift_fermi=True)

will shift the two Fermi energies to 0, so that the two bands overlap each other.