Returning Alpha Map from 'ExcessMapEstimator'

[matthew-w-lundy]

Is your feature request related to a problem? Please describe.
Relating to #5245, identifying problems relating to acceptance in ExcessMapEstimator is difficult because only returns a FluxMap which contains no alpha information.

Describe the solution you'd like
I would like the ability to produce an alpha map with the output of ExcessMapEstimator.run().

Describe alternatives you've considered
You can generate an implied alpha map using the TS map and the On, and scaled Off but this is rather clunky and requires a user defined inverse significance function.

Additional context
Alpha is often a critical tool for assessing issues and uncertainties in significance calculations. Returning this information as a diagnostic immediately highlights potential issues when a user has selected parameters that contain issues (i.e. too small a width in their ring background). It feels clunky to add this information to the 'FluxMap' because you should be able to generate maps and significances where alpha isn't a significant factor in the calculation of the background. It could be just an optional return of 'ExcessMapeEstimator' which is what I currently do.

[github-actions]

Merci! We will respond to your issue shortly. In the meantime, try import gammapy; gammapy.song()

[registerrier]

Thanks @matthew-w-lundy . This would useful indeed. Do you have a prototype/example implementation?