Updates model.rst to include info on GModelSpectralGauss
Correct Doxygen documentation
Update for latest changes
Adjusted model documentation.
first attempt to improve the solid angle method in GWcs::solidangle
Compute polygon inner angles in GWcs::solidangle method using spherical triangles.
Optimize solid angle computation.
Makes correction to GWcs::solidangle
Updates docstring to reflect changes made to  GWcs::solidangle
Reformat code
Implement special cases where pixel is at pole for solidangle() method.

When a pixel touches the pole, the pixel is effectively a triangle and not a polygon. This case is now handled properly.
Update following implementation of precise solid angle computations (feature #1017)
Merge branch 'issue_1017' into issue_1017_jk

Conflicts:
	src/sky/GWcs.cpp
Adjust test result reference values after improving precision of GWcs::solidangle() method.
Merge branch 'issue_1017_jk' into integration

Conflicts:
	ChangeLog
Use Huilier's theorem to compute the spherical excess in the solidangle() method.

The former trigonometric method led to different results on different platforms due to numerical imprecisions. We now use Huilier's theorem to be less sensitive to numerical problems. The polygon is decomposed in two triangles for this purpose.
Adapt reference values to new GWcs::solidangle method using Huilier's theorem.
Correct solidangle computation for triangular pixels.