Use FluxMaps in TSMapEstimator

[adonath]

Description

This pull request implements the use of FluxMaps in the TSMapEstimator and adds the additional quantities defined in #3278.

• For now I added support __getitem__ in FluxMaps for backwards compatibility with the dict of maps. Keep it?
• Add FluxMaps.niter?
• Add unit tests for additional quantities

[codecov]

Codecov Report

Merging #3285 (b829da8) into master (acba5b5) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3285      +/-   ##
==========================================
+ Coverage   93.79%   93.81%   +0.01%     
==========================================
  Files         144      144              
  Lines       17854    17857       +3     
==========================================
+ Hits        16747    16753       +6     
+ Misses       1107     1104       -3     

Impacted Files	Coverage Δ
gammapy/estimators/core.py	94.79% <100.00%> (+1.85%)	⬆️
gammapy/estimators/flux_map.py	92.08% <100.00%> (ø)
gammapy/estimators/flux_point.py	92.94% <100.00%> (-0.03%)	⬇️
gammapy/estimators/ts_map.py	97.99% <100.00%> (-0.05%)	⬇️
gammapy/modeling/parameter.py	96.32% <100.00%> (+0.02%)	⬆️

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