Replace temporary element names with their assigned names

[T0mstone]

The definitions.units file still contains the old temporary names for the elements 113, 115, 117 and 118, see here. These have been given official names by now: Nihonium, Tennessine, Moscovium and Oganesson.

If possible, the file should be updated to include the correct names and their atomic symbols (Nh, Mc, Ts, Og).

Remark

The strings Mc and Ts currently have meaning (10⁶ times the speed of light and 10¹² seconds respectively). I'm not sure how this would be resolved, but a similar conflict already exists with kelvin and potassium, which is currently handled in that K always resolves to kelvin (as it seems).