Create a struct to represent of species in multiple aerosol models and phases

[boulderdaze]

As we introduce supporting multiple aerosol models and phases (e.g. aitken, accumulation mode, aqueous phase, etc.), species in the state variables need to specify both their model characteristics and their phase. For example, aitken.aqueous.SO2.

Struct could represent species with the model characteristics (if exists,) and phases.

• Aerosol model or mode (e.g. aitken)
• Phase (e.g. aqueous)
• Species name (e.g. SO2)

Consider how to communicate species name with the host model, which may not have knowledge of specific aerosol models or phase.

// Solver state variables
gas.H2SO4
primary.number_concentration
primary.aqueous.H2O
primary.aqueous.H2SO4