Move SafeMaskMaker and adapt mask_safe handling in MapDatasetMaker

[adonath]

This PR implements a few changes related to the SafeMaskMaker and mask safe handling in general:

• Move SafeMaskMaker to gammapy/cube
• Remove offset mask application from MapDatasetMaker
• Implement offset-max mask option in SafeMaskMaker
• Adapt tests and tutorials

[codecov]

Codecov Report

Merging #2538 into master will increase coverage by 0.04%.
The diff coverage is 97.93%.

@@            Coverage Diff             @@
##           master    #2538      +/-   ##
==========================================
+ Coverage   91.41%   91.45%   +0.04%     
==========================================
  Files         144      144              
  Lines       16297    16297              
==========================================
+ Hits        14898    14905       +7     
+ Misses       1399     1392       -7

Impacted Files	Coverage Δ
gammapy/cube/psf_map.py	95.57% <100%> (+0.12%)	⬆️
gammapy/cube/background.py	79.41% <100%> (ø)	⬆️
gammapy/maps/wcsnd.py	95.4% <100%> (+0.02%)	⬆️
gammapy/cube/fit.py	89.55% <100%> (-0.16%)	⬇️
gammapy/spectrum/make.py	97.05% <100%> (+2.16%)	⬆️
gammapy/analysis/core.py	94.21% <100%> (+0.05%)	⬆️
gammapy/cube/edisp_map.py	95.65% <100%> (+0.11%)	⬆️
gammapy/cube/make.py	96.81% <96%> (-0.85%)	⬇️
gammapy/catalog/fermi.py	94.63% <0%> (-0.01%)	⬇️
gammapy/maps/wcs.py	96.9% <0%> (ø)	⬆️
... and 2 more

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