Updates to the photon CR in view of the stat analysis

This MR contains changes needed in order to produce the correct templates to be used in the statistical analysis.

Mainly, the important script is build_asimov_templates, which builds the Asimov templates from "regular" templates produced in the analysis.

Several other changes are also made in order to avoid potential Combine problems (notably with empty or negative bin contents).

Other small changes are also inicluded, to keep the main repo aligned with my current working version.