Add CleaverExtension extension

Cleaver - A MultiMaterial Tetrahedral Meshing Library and Application

The Cleaver Library is based on the 'Lattice Cleaving' algorithm:
Bronson J., Levine, J., Whitaker R., "Lattice Cleaving: Conforming
Tetrahedral Meshes of Multimaterial Domains with Bounded Quality".
Proceedings of the 21st International Meshing Roundtable (San Jose, CA,
Oct 7-10, 2012)

The method is theoretically guaranteed to produce valid meshes with
bounded dihedral angles, while still conforming to multimaterial
material surfaces. Empirically these bounds have been shown to be
significant.

The extension is a CLI for Slicer. You can select input files, a
number of options, and output format for the tetrahedral mesh.