Refactor task chain up to limits

This PR refactors the task chain to create limit plots. Most important changes:

• Added MultiDatacardBaseTask, POITask1DWithR and POIScanTask1DWithR
• Encoded hash of input datacards into output paths
• Added two new plots (PlotMultipleUpperLimits, PlotUpperLimitsAtPOI) for comparing limits across configurations
• Updated docs accordingly