Adding mesh cell inertia calc.

- easier to clean, avoid confusion with 'real' applications, etc.

Exception: applyWallFunctionBoundaryConditions.C cannot split #include
directives.

in a similar way to calculate the inertia of cells.

triangle, from:

http://en.wikipedia.org/wiki/Inertia_tensor_of_triangle

Adding the ability to calculate the inertia tensor of a polygon face
by summing the triangle inertias.

Added a test application to draw a test face with its principal
axes and moments of inertia.

triangle, from:

http://en.wikipedia.org/wiki/Inertia_tensor_of_triangle

Adding the ability to calculate the inertia tensor of a polygon face
by summing the triangle inertias.

Added a test application to draw a test face with its principal
axes and moments of inertia.