Pypartmc

- removed `fake_mpi` (partmc codebase is now compiled without MPI support)

- add `solute_kappa` property to `AeroParticle`
- remove hardcoded relative humidity in `EnvState` (now takes the value from ctor-arg json)
thanks zdaq12!

- include call to `fractal_set_spherical` in `AeroData` ctor and remove now unneeded fractal parameter settings in multiple locations in tests (thanks zdaq12 )

publishing Python 3.7, 3.8 & 3.9 packages for macOS

statically link libgfortran on macOS to address lack of libgfortran.dylib on default brew gcc installs

- `condense_equilib_particle`, `loss_rate_dry_dep` and `loss_rate` functions (kudos zdaq12)
- new `AeroData` methods/properties: `__len__`, `frac_dim`, `vol_fill_factor`, `prime_radius`, `rad2vol`, `vol2rad`, `diam2vol`, `vol2diam` (kudos zdaq12)
- new class: `AeroParticle` with the following methods/properties: `volumes`, `volume`, `species_volume`, `dry_volume`, `radius`, `dry_radius`, `diameter`, `dry_diameter`, `mass`, `species_mass`, `species_masses` (kudos zdaq12)
- new methods for `AeroState`: `total_num_conc`, `total_mass_conc`, `num_concs`, `masses`, `volumes`, `dry_diameters`, `diameters`, `crit_rel_humids`, `mixing_state`, `bin_average_comp` (kudos jcurtis2)
- new `EnvState` methods/properties: `set_temperature`, `temp`, `rh`, `height`, `pressure` (kudos jcurtis2)
- new CAMP-related classes CampCore & Photolysis (for now without any logic exposed, just to make `run_part` understand all arguments neded when CAMP is compiled in)