improve convergence.html: add colored range for surplus tolerance
It would be great if the surplus plots could contain the tolerance threshold (p80_surplusMaxTolerance) as light colored background, similar to the way it is done in the convergence plots (but it doesn't need to be differently colored to show if the target is achieved or not, just as a fast visual cue what the target level in this run was)
this would be relevant for all the surplus plots under the "surplus" header, but also the surplus plots in the "prices and surplus in one plot"