Allow user to change the unit system

[jsantner]

The program currently uses chemkin's default units (MKS). Implement a menu to allow user to choose any combination of distance, energy, mass (or mole), temperature, and pressure units.

[jsantner]

Fixed in f2048ae. Still need to test more - there are some differences, that seem larger than rounding error. Also, still need to add some other units, like psia