Update task parameters to fit most use cases

This PR updates the names and formats of some task parameters to better fit the needs of the inference tasks. In particular, the changes introduce the clear distinction between actual POIs and parameters that are scanned.

For instance, in the current master, some tasks have parameters --poi and --r-poi, meaning that the --r-poi is the actual POI that is free in the fit, whereas --poi is actually the parameter that is scanned (e.g. kl). This is confusing and obfuscates what is done by combine in the background. This PR is intended to correct this.

The changes will be presented in detail in the next HH working meeting on 19.1.2021.

Additional features:

• Postfit S-over-B plot, merged over all histogram bins in the datacards.
• 2D exclusion plots of "scan parameters" (e.g. kl and kt) given a POI (e.g. r or r_gghh).
• Move tasks into files within tasks/ with descriptive names.
• Proper error propagation of theoretical uncertainties in plots.

Things left to do:

• Add documentation to 2D exclusion plot.
• Check if plotting needs to be updated given the changes to the HHModel by Luca and Fabio.
• Update external documentation. Will be updated in an other PR.

@mfackeld