Allow a user specified list of k-points for the band structure calcul…

…ation.

Add input tags `explicit_kpath` and `explicit_kpath_labels`
If `explicit_kpath_labels` and `explicit_kpath` are present in input,
the band interpolation code will use an explicitly provided list of kpoints
specified by `explicit_kpath`, instead of using `bands_num_points` to auto generate a list of kpoints.
This allows a finer control of the list of kpoints to be interpolated and
direct comparison with PW eigenvalues.

Co-authored-by: Junfeng Qiao <[email protected]>