Fem cube: precise the node ordering rule and write UnitTest to demonstrate it.

The unit test goes over all the cases (correct and incorrect).
For each incorrect one, an ASSERT is triggered, printing a message on the console.
So, that is a lof of print for 1 unit test.
Math: Gauss-Legendre doxygen doc more Latex-ish
ExampleFem3D clean the example (more space between obj + remove 4th column)
ExampleFem3D remove global variables by creating a Cube class.