ndrop_bam cleanup (answer-changing): pres calculation

During the review of https://github.com/ESCOMP/atmospheric_physics/pull/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`

Start with the ndrop_bam implementation and the review discussion in PR 411, focusing on `pres = rair*rhoair*tair`. Done means reconciling that calculation with the cited paper and the project's provided-pressure conventions, then agreeing on a change that avoids unintended answer differences.

Written by the indexing model from the issue text.