Tetrahedron integration

• P. B. Allen, Phys. Stat. Sol. (b) 120, 629 (1983)
• Blöchl et al, Phys. Rev. B 16, 232 (1994)

• Add introduction to README
• Add references
• Full coverage
• Docs
• Improve gaussian with small e2-e1, e3-e2, e4-e3, Add tests
• Implement Blöchl correction (Eq. (22))
• Which diagonal to cut the cube into tetrahedron
• Modularize (e0, v0) -> (e1, ..., e8)
• Add type for piecewise polynomial (?)
• Functions other than gaussian (by quadrature integration?)