Compare sensitivity calculated from DL2 events with sensitivity derived from IRFs

[maxnoe]

Another validation step that could be done is comparing the two ways of computing a sensitivity to make sure that sensitivity computation and IRF computation are consistent.

So compute sensitivity on DL2 events with the sensitivity derived from IRFs calculated from the same DL2 events.

[maxnoe]

This was done by @TarekHC and @JBernete and the results agree to a reasonable account.

We should add a test though if possible.