streamline chemical potential interface: get_dmft_bands does not
return dft_mu anymore, rename dft_mu -> tb_mu. tb_mu is subtracted
from TB model and can be later added to the spectral function
again when DMFT has been run with removed tb_mu.
additional parameter mu_shift can be used to tune the final mu for the
calculation of the spectral function
when calculating fermi slices tb_data holds now the full 2D kpoint
mesh correctly
eigenvectors are now also returned in tb_data when band_basis=True
allow to pass extra projections, i.e. for band unfolding, to be
applied when band_basis=True to A(k,w).