ENH: Unit test for vtkMath

A comprehensive test for vtkMath.

This UnitTest provides 86% coverage for vtkMath. When combined with
TestMath, the total coverage comes to over 96%. The remaining untested
lines are special cases that I have not been able to exercise.

During the test creation process, these bugs were found:

NextCombination() always returned after one interation.

Memory leak in SolveLeastSquares

Change-Id: I22dfd2198e52f8980445e0ba5e951f811700af16
Merge topic 'Coverage_Math' into master

5988b551 ENH: Unit test for vtkMath