Default energy range in Bands widget

[cpignedoli]

Is your feature request related to a problem? Please describe.
For large systems, the bandsplot by default shows an almost unreadable set of bands: too many bands.

Describe the solution you'd like
By default, show the energy range containing the top 8 valence bands and bottom 8 conduction bands or less if less bands were computed

Describe alternatives you've considered
We could fine tune with experience

Additional context