Consolidate modal averages

[pbosler]

Computations like modal volume, hygroscopicity, and particle size require averages over modal mass- and/or number-mixing ratios. These quantities are used in many aerosol processes (they have already shown up in gasaerxch, calcsize, and water uptake, for example). Instead of repeating the same computations in several places, we should consolidate such things to a single set of functions.

[jeff-cohere]

Our "direct port" strategy unfortunately precludes this. But any future cleanup we do on the code should make a better attempt at reusing implementations and not re-implementing them everywhere.