Reimplementing the uniform density solid inertia calculation from:

http://www.geometrictools.com/LibPhysics/RigidBody/RigidBody.html

in surfaceInertia utility.  Allows calculation of shell or solid
properties of a triSurface, with optionally specified volumetric or
surface density.  Default behaviour is to calculate inertia around
centre of mass, with option to specify reference point.

Various test surfaces supplied and tested against analytical results.