kpenvelope 0.4.0: subband character analysis

This release adds band_character, dominant_character and character_vs_k: the heavy-hole, light-hole and crystal-field split-off composition of each envelope state, and its evolution along an in-plane path. Band mixing is the physics behind the one subband, many masses problem this package makes explicit: a state that is pure HH at the zone center picks up LH and CH weight as the in-plane momentum grows, and its local mass moves with that composition. The tests assert the exact zone-center block structure of the six-band Hamiltonian (at kt = 0 only LH and CH couple, through delta3, so every zone-center state has HH fraction exactly 1 or exactly 0), machine-precision normalization of the fractions, the small CH admixture of LH states that the delta3 coupling demands, and the onset of mixing at finite in-plane momentum. Basis convention follows Chuang and Chang, Phys. Rev. B 54, 2491 (1996).