Pypartmc

- added urban plume simulation example notebook, kudos jcurtis2!

- support for time-varying aerosol specs in Scenario (`aero_background` and `aero_emissions`)
- new properties of `Scenario` class: `aero_emissions`, `aero_emissions_n_times`, `aero_emissions_rate_scale`, `aero_emissions_time`, `aero_background`, `aero_dilution_n_times`, `aero_dilution_rate`, and `aero_dilution_time`
- phased out test-time-requirements.txt (in favor of [tests] variant for setup.py-defined dependencies)
- moved example notebooks into the PyPartMC repository

- new properties and methods in `AeroParticle`: `moles`, `mobility_diameter`, `density`, `approx_crit_rel_humid`, `crit_rel_humid`, `crit_diameter`, `coagulate`, `zero` and `set_vols`

Kudos zdaq12 !

- support for optionally linking against Model for Simulating Aerosol Interactions and Chemistry (MOSAIC, closed-source software from PNNL)

Kudos jcurtis2 !

- added default values for `AeroState::dist_sample` arguments: sample_prop, create_time, allow_doubling & allow_halving (thanks jcurtis2)

- made `GasState::mix_rats` a read-write property (previously was read-only) - kudos jcurtis2