Add unit tests on primitive's geometries

[lbinria]

Add robust tests on:

• [x] Triangle2 gradient
• [ ] Triangle3 gradient
• [ ] Tetrahedron gradient
• [x] Wedge volume / bary_verts
• [x] Poly extraction on volume with different facet shapes
• [x] Tetrahedron bary_coords
• [x] Segment 2
• [ ] Segment 3

Tests should:

• Generate random primitives (regular, inversed, degenerated)
• Compute an expected value and compare with actual value