NOx city emission paper

https://doi.org/10.5194/egusphere-2023-346. Significant updates in Level3_List class; Use cases in city_emissions.py and basin_emissions.py

GRL paper (10.1029/2022GL101102)

v0.2

iasi and others

ACP RRNES paper

Merge pull request #4 from Kang-Sun-CfA/remove-mutable-defaults

Remove mutable default args from functions in popy.py