Residual Gas

This is partly related to the #51 (closed), but more specifially, I'm wondering how to include multiple molecular species.

We want to have H20, N2, O2, and H2 at least. Since they each have different masses and polarizabilities and partial pressures, they will each have a different curve (different DC level and different rolloff frequency).

Later on down the road, we probably also want to include the partial pressure for each species as a function of position along the beam tube, since the beam size changes as a function of position within the cavities.

I'd recommend using a separate field for the pressure for squeeze film damping since that is more likely sourced by outgassing near the mirror and would not effect the usual residual gas phase noise.