Improve the perfomance of solid angle calculations

[Nick Draper]

Use the triangulation to improve the accuarcy and performance

[Ronald Fowler]

(In [1946]) Updated solid angle calculation using the surface triangulation. If the triangulation is good and the number of elements not too high this gives a better and faster estimate of the solid angle. Added unit tests. Re #401

[Ronald Fowler]

(In [1951]) Fix to fall back to ray tracing solid angle if no triangulation found. Also fix to Nexus not to print error message if "units" attribute not present. Re #401

[Ronald Fowler]

(In [1953]) Another minor fix for the fix, for Linux in the header file Re #401

[Ronald Fowler]

(In [1957]) Fix to not call glIsList when displaylist is 0. re #401

[Ronald Fowler]

(In [1958]) Updated solid angle calculation for cuboids (marked as such in XML) to use faster and more accurate triangle method. Also updated unit test solidAngle to expect new values. For the relatively high aspect ratio cuboids in the example, about 10 wide to 1 long, the ray tracing method is only accurate to 15% which is not good. Re #401

[Martyn Gigg]

(In [1959]) Small fix to template arguments so that Mantid will compile on Linux. Re #401

[Sri Nagella]

(In [1967]) refs #401 Fixed Object solid angle calling initDraw

[Stuart Campbell]

This ticket has been transferred to github issue ​1249