Psmap implementation in fermipy

[omodei]

This PR implements the psmap implementation in fermipy (wrapping the code gtpsmap.py from FSSC (author Philippe Bruel). In particular:

1. The class PSMapGenerator is implemented as mixing class for GTAnalysis. It provides the method psmap that wrap the code gtpsmap and return the usual dictionary with relevant outputs
2. AnalysisPlotter.make_psmap_plots provides the same interface of make_tsmap_plots or make_residmap_plots to plot pamaps.
3. Added relevant documentation in the "Advanced Analysis Methods" tab, with class description and example.
4. This PR also fix the broken API documentation
5. This PR fix the random NaN error raised by spicy while scanning the parameters, when the first value of the array is 0

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.12360% with 84 lines in your changes are missing coverage. Please review.

Project coverage is 49.29%. Comparing base (bc2aa7d) to head (947b6d6).
Report is 5 commits behind head on master.

Files	Patch %	Lines
fermipy/gtpsmap.py	77.16%	79 Missing ⚠️
fermipy/psmap.py	93.33%	3 Missing ⚠️
fermipy/plotting.py	97.36%	1 Missing ⚠️
fermipy/utils.py	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #578      +/-   ##
==========================================
+ Coverage   48.73%   49.29%   +0.55%     
==========================================
  Files         141      143       +2     
  Lines       26520    26959     +439     
==========================================
+ Hits        12925    13289     +364     
- Misses      13595    13670      +75     

Flag	Coverage Δ
unittests	49.29% <81.12%> (+0.55%)	⬆️

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[ndilalla]

Good to go!