Implemented GModelSpatialEllipticalDisk and fixed bug in GModelSpatialElliptical
Merge branch '761-implemented-GModelSpatialEllipticalDisk' into integration

Conflicts:
	configure.ac
Review code before integration.

I quickly went over the code before integration to make it compliant to the coding standards. Most of the modifications were cosmetical (related to code formating)

In GModelSpatialEllipticalDisk::eval() I removed the std::pow(..., 2) calls and replaced them by simple multiplications as the later are much faster. This should speed up the computation somewhat. More  importantly, I added an initial check on theta to see whether the position is located within the enclosing circle of the ellipse. This will simply return 0 for any locations outside the circle, avoiding unnecessary computations.

In GModelSpatialEllipticalDisk::theta_max(), I added a check on whether major() or minor() are the larger value, and based on the check, I return the larger of both. This is important as a model fit may make minor() larger than major().

In GModelSpatialEllipticalDisk::read() and GModelSpatialEllipticalDisk::write(), a few bugs have been corrected.
Review code before integration.

I quickly went over the code before integration to make it compliant to the coding standards. Most of the modifications were cosmetical (related to code formating)
Replace minor() by semiminor() and major() by semimajor().

The minor() and major() methods produced problems on FreeBSD where both functions are predefined macros.