Implement HpxNDMap.to_region_nd_map

[adonath]

Description

This pull request implements HpxNDMap.to_region_nd_map and HpxGeom.region_mask. This allow to extract a RegionNDMap from a HpxNDMap using various methods.

[codecov]

Codecov Report

Merging #3308 (fb15722) into master (75398b6) will decrease coverage by 0.03%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #3308      +/-   ##
==========================================
- Coverage   93.85%   93.81%   -0.04%     
==========================================
  Files         144      144              
  Lines       17968    17998      +30     
==========================================
+ Hits        16864    16885      +21     
- Misses       1104     1113       +9     

Impacted Files	Coverage Δ
gammapy/maps/hpxnd.py	94.38% <77.27%> (-1.07%)	⬇️
gammapy/maps/hpx.py	88.32% <87.50%> (-0.01%)	⬇️
gammapy/modeling/iminuit.py	92.39% <0.00%> (-3.27%)	⬇️

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