PartMC interface: per-gridcell aerosol state and sector/natural-source emissions

[jcurtis2]

This PR adds the following:

• Per-gridcell aerosol state: an aero_state is now allocated and time-stepped for every (column, level), initialized from the first-timestep MAM constituent fields.
• Sector-resolved anthropogenic emissions: mapping each present (MAM-group, CMIP6 sector) pair (AGR, ENE, IND, RCO, SHP, SLV, TRA, WST) to a PartMC log-normal emission mode, sourced from the sector-resolved surface-emission inventory.
• Natural-source emissions: sea salt and dust are emitted as bin-resolved sampled modes: sea salt from the sslt_sections flux polynomials, dust from the MAM dust_emis mass flux subdivided into log-spaced sub-bins per MAM bin.
• A legacy cflx-based emission pathway is retained behind the use_sector_emissions toggle. This will emit based on the MAM modes with their collapsed mixing state.

Summary of actual changes to files:

• partmc_interface/partmc_mod.F90: per-gridcell state, emission sector building, sector and natural-source emission inputs mapped to PartMC modes, and initial conditions mapped to modes.
• chemistry/mozart/mo_srf_emissions.F90: accessors for sector names and per-sector surface fluxes
• modal_aero/dust_model.F90, modal_aero/aero_model.F90: expose dust and sea salt information for creating natural emissions.