Allow scenarios to be directly compared

[brynpickering]

Often, we want to compare results between scenarios. Perhaps there could be a scenario comparison tab, that allows a simplified set of scenario comparisons (since loading e.g. timeseries data for multiple models will be quite heavy on compute). The user could then do something like calligraph path/to/result.nc --compare-to path/to/scenario1.nc path/to/scenario2.nc ... to set their baseline and their scenarios to which that baseline will be compared where possible.