Add formulae for physical chemistry concepts

[bjodah]

chempy:

• [x] Einstein–Smoluchowski relation
• [ ] DLVO theory
• [ ] Fokker-Planck/Smoluchowski/Nernst-Planck equation, for now refer to chemreac
• [ ] Maxwell–Boltzmann distribution

chempy.thermochemistry

• [ ] Shomate equation
• [ ] Data equivalent to NIST-JANAF.

chempy.electrochemistry

• [x] Nernst equation (see #36)

chempy.radiation

• [ ] Bethe formula

chempy.kinetics:

• [ ] Langmuir-Hinshelwood kinetics
• [ ] Smoluchowski_coagulation_equation
  • J.A.D. Wattis / Physica D 222 (2006) 1–20
• [ ] Bateman Equation (i.e. refer to https://github.com/bjodah/batemaneq)