.tet file reader

• Currently in imstk, only .veg is supported for volumetric mesh, surface is then extracted.
• .tet file comes with links info( the barycentric coordinate for an associated surface mesh), so there is no need to extract, and the surface mesh can be enclosed inside the “real surface” of volumetric mesh.
• More importantly, in some scenario, we want to use a coarse volumetric mesh to drive a much finer surface, the links info make it possible.