Add option to change the density of k-points in the band structure calculation

**Is your feature request related to a problem? Please describe.**
I wanted to use a fast protocol, but still have a nice band structure with a dense mesh of k-points along a path. However, the number of k-points is set by the protocol and I don't see a way to change it.

**Describe the solution you'd like**
Have, in the bands settings, the possibility to change the k-points density (similar to what is possible in the advanced settings for the SCF k-points density)