ndrop_bam cleanup (answer-changing): pres calculation

[jimmielin]

During the review of #411, @nusbaume originally wrote:

I wouldn't do anything about this now (as it would change answers), but it makes me a little nervous that this is calculating its own pressure instead of just using the provided pressure (e.g. air_pressure).

At some point it might be good to double-check the paper and these equations to make sure everything is consistent with the rest of the physics (e.g. it's not using dry air pressure when expecting total air pressure), but for now maybe just add it to the "future work" issue?

The relevant code is pres = rair*rhoair*tair