exo-cesm/CESM2.1.3: CESM2.1.3 edits for oxygenated Earth WACCM6 simulations

This release covers code edits for three papers Cooke et al. 2022, Cooke et al. 2023, and Cooke et al. 2026.

The lower boundary mixing ratio for oxygen was modified between 0.1% the present atmospheric level (PAL) to 150% PAL. These simulations were written up as research articles regarding lower ozone estimates, reflection spectroscopy predictions for Earth-like analogue exoplanets, and hydrogen escape since the Great Oxidation Event.

The repository contains information and code modifications required to modify CESM2.1.3, specifically WACCM6, to reproduce the work.