The CESM 2.1 release will use CMIP6 tunings by default, but needs to be able to
reproduce the CESM 2.0 results as well. To do that, we introduced multiple
default settings files and a mechanism to select which is used. The default is
to use settings_latest.json, but there is also a settings_cesm2.0.json file.

default_diagnostics has also been renamed to diagnostics_latest, but at this
time MARBL does not support multiple possible default diagnostic files.