Plotting uncertainty groups and variations as filled bands

[reimersa]

• Adds argument to plot grouped variations, just like --varNames but using the group names (e.g., --varGroups resum)
• Adds option to plot variations as filled bands with customizable opacity
• Needs Allow alpha to propagate from original handle wums#20 for proper markers in the legends

[cippy]

Is it intended that it only uses global impacts? Usually the code produces nominal impacts by default, unless rabbit_fit is called with --globalImpacts, so the option --computeHistImpacts might not always reflect the intended behaviour (or are both nominal and global uncertainty bands stored?)

[reimersa]

In fact --computeHistImpacts of rabbit_fit.py propagates the global impacts, so only those are stored as histograms that can be plotted. This makes sense from a plotting point of view because the error bands will sum up in quadrature to the total uncertainty in the prediction then.

[lucalavezzo]

lgtm thanks @reimersa