ResultsHandler: load TS distribution from different analysis path

[mlincett]

In the standard workflow, flarestack runs n_trials per injection step and 10 * n_trials for the background (where 10 is actually configurable).

But sometimes one runs different injection setups (different spectral indices, different energy limits) for the same analysis, that means the background TS distribution is unchanged.

It is possible to run only signal trials (background_ntrials_factor = 0) but then one has to manually manipulate the ResultsHandler attributes in order to plug in the background TS distribution from a previous analysis (this was done in the past).

As a better design, ResultsHandler could take an analysis name (path) as a keyword to use as a source for the background TS.

Ultimately, one may even choose to just run the background as a separate job, and produce only signal injections for the different scenarios. This is already possible, but to be useful ResultsHandler must be modified to support this use case.

[JannisNe]

This is a good point. In my analysis I did the manual ResultsHandler manipulation as you mention.
Running background trials separately is actually what other analysis frameworks do and I think it makes sense.