Add hygroscopicity to Species and Diagnostic variables

[pbosler]

Hygroscopicity

Hygroscopicity quantifies each species' affinity for adhering to liquid water particles. It has two components: A base value that is a material property of a species, and a modal average that relates to the well-mixed assumption of species within a mode. We therefore need to add two things to Haero (and associated tests, ofc) to account for hygroscopicity.

Species' base values

• [x] Add hygroscopicity (in SI units) to the Species class, with default values for the MAM4 configuration. Later, we'll need the capability to look up values from data, to support non-standard species, but this is likely a task for NcReader that can be put off until we have a need.

Modal average values

• [x] Add modal average hygroscopicity as a Diagnostic variable. See https://github.com/eagles-project/MAM_documentation/tree/master/water_uptake equations (7) and (8).

NOTE: the formulas in the MAM Documentation are written in terms of (a) dry air and (b) molar mixing ratio (not mass mixing ratio).

[pbosler]

Moved the second checkbox to a new issue, #143