Varying atm option fixes rho at 1d-10

[Debraheem]

I wouldn't call this a bug, but at the Massive star Hackathon @rhdtownsend identified that we are using a fixed density floor for the outer_BC in the atmosphere integration routine when atm_T_tau_opacity = 'varying':

mesa/atm/private/atm_t_tau_varying.f90

Line 277 in 1847457

real(dp), parameter :: RHO_OUTER = 1.E-10_dp

It's not clear why this value was chosen, but reducing it to a lower density might be warranted, and even necessary for some models where the surface extends beyond 1d-10 g/cm^2 in density. It remains to be seen.

[rhdtownsend]

Ideally, we would want to use the smallest density supported by the opacity tables. Any way to easily compute that?

[Debraheem]

I think it's doable, I'm just waiting to address this until I resolve #997, since the eos in this territory would otherwise be coming from helm and be that of an ionized gas mixture at lowT.