Add QC plot showing b values and shell assignments

[tsalo]

Summary

Yesterday, we discussed adding a plot to show the distribution of b-values in a scan and how they're assigned to different shells (when applicable).

Next steps

I took a crack at it with some fake b-values going from 5 to 3005, but I don't think it looks great.
I used vertical spans to show the shells and a histogram to show the b-values. The problem is that the values are very closely spaced in a very large range overall, so if we want bins to be reasonably sized then different b-values will already be bundled together.