Add FoVBackgroundMaker class

[registerrier]

Description

This pull request introduces a FoVBackgroundMaker class. It is very simple. It takes an exclusion mask on __init__. FoVBackgroundMaker.run(dataset) simply performs a fit of the model. The assumption here is that it only consists of the background_model.

Tests have been included, in particular one checking that in case of non-convergence the parameters are the initial ones. In this case a simple log.warning is raised. (this should be tested properly). It is not clear that this is the best behavior. Because ideally the dataset should be excluded.

Dear reviewer

please comment on the choice of for non-convergence.

[codecov]

Codecov Report

Merging #2697 into master will increase coverage by 0.04%.
The diff coverage is 98.03%.

@@            Coverage Diff            @@
##           master   #2697      +/-   ##
=========================================
+ Coverage   91.66%   91.7%   +0.04%     
=========================================
  Files         142     143       +1     
  Lines       15988   16040      +52     
=========================================
+ Hits        14655   14710      +55     
+ Misses       1333    1330       -3

Impacted Files	Coverage Δ
gammapy/cube/__init__.py	100% <100%> (ø)	⬆️
gammapy/cube/fov.py	98% <98%> (ø)
gammapy/cube/fit.py	89.65% <0%> (ø)	⬆️
gammapy/maps/wcs.py	96.58% <0%> (ø)	⬆️
gammapy/modeling/iminuit.py	95.5% <0%> (+4.49%)	⬆️

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[adonath]

Thanks a lot @registerrier, I've left one minor inline comment concerning the fit method.

I think we should also add an example somewhere how to use this class, either in docs/cube/index.rst or maybe analysis_2.ipynb.

Otherwise looks good to me. Please merge whenever you think its ready.

[registerrier]

Modifications to the tutorials will be made in a further pull request.
All tests pass. Merging.