Default energy range in Bands widget

**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**