Vertical integral algorithm

[taobrienlbl]

We need an algorithm for calculating vertical integrals: both on native model levels and on pressure levels. #354 will close this.

This may depend on #353, since the pressure level calculation may need surface variables (e.g., surface specific humidity) for an accurate calculation; these are usually stored in separate files from height-dependent versions.

[burlen]

@taobrienlbl I would like to get the data necessary to incorporate surface pressure. Can you share an instance?

[taobrienlbl]

@burlen - it should be that simple, but unfortunately it isn't. Most models didn't supply surface pressure; they instead supplied sea-level pressure, which is the pressure the surface air would have if brought down to the elevation of the ocean surface.

So we actually have to add an algorithm to infer surface pressure from sea-level pressure, which is a bit of a longer conversation.