Add units: mass and molar mass of elementary particles.

[crystal-growth]

Adding CODATA values of elementary particles masses and molar masses. Tests added to MolarMass, CODATA values had precision insufficient to satisfy the assert_approx_eq, so conversion coefficient were calculated as: particle_molar_mass = mass * Avogadro_constant. CODATA coefficients are given in comments.

[iliekturtles]

Skipping this one for the moment (partial changes at https://github.com/iliekturtles/uom/tree/add_elementary_particle_mass_units). I want to dig deeper and find out why the real CODATA values have precision issues.