Skymap inconsistency fixed.

[ndilalla]

Fixed some inconsistencies and bugs in the skymap library:

• Use tuple indexing (instead of list indexing) so advanced indexing matches the array layout for 2D and 3D maps
• Return shape aligned with HEALPix, so that the result is now returned as (ne, np) (energy × sky points) instead of (np, ne), so WCS and HEALPix get_map_values() have the same convention
• Updated docstrings to better explain that the internal array is (energy, lat, lon) (and (lat, lon) for 2D)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.78%. Comparing base (92cf94f) to head (9f6f897).

Files with missing lines	Patch %	Lines
fermipy/skymap.py	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #658      +/-   ##
==========================================
- Coverage   50.79%   50.78%   -0.01%     
==========================================
  Files         143      143              
  Lines       27265    27269       +4     
==========================================
  Hits        13849    13849              
- Misses      13416    13420       +4     

Flag	Coverage Δ
unittests	50.78% <0.00%> (-0.01%)	⬇️

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