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