Calculate sensitivity at non-zero offsets and validate against EventDisplay

[maxnoe]

General discussion of the points that came up here #194.

Up to now, we only produced sensitivity curves using point-like gammas coming from the center of the field of view. Doing it for multiple offsets using diffuse gammas requires doing some steps differently and should also be validated against EventDisplay results.

This is different from calculating IRFs at multiple offsets, the main difference being that calculating IRFs in principle does not require rewaiting the diffuse gamma ray simulations to a physical point source flux, which requires making some assumptions.

IRFs at different offsets (full enclosure and point-like) are already supported and tested e.g. by the LST analysis.

The procedure I imaging, and maybe @GernotMaier can comment on the EventDisplay approach is:

Count events in bands of the field of view offset (i.e. rings around the center of the FoV) essentially integrating the diffuse flux over the solid angle in that ring and then weight them to the Crab Flux, as shown as an example here:
#194 (comment)