Allow all concentrations (in both initial and evolving conditions) to be specified in mixing ratio units (mol mol-1, ppth, ppm, ppb, ppt) and use the air density to convert to concentrations in mol m-3.

Acceptance Criteria

• Any species concentration in the initial conditions or evolving conditions tables can have units of mol mol-1, ppth, ppm, ppb, ppt
• Concentrations in mixing ratio units are converted to mol m-3 prior to running a simulation

Ideas

• multiply mixing ratios in mol mol-1 by the air density (mol m-3) to get concentrations in mol m-3
• mol mol-1 = ppth * 1.0e-3
• mol mol-1 = ppm * 1.0e-6
• mol mol-1 = ppb * 1.0e-9
• mol mol-1 = ppt * 1.0e-12