Replace CO2 range check ERROR with WARNING

[TomasTorsvik]

Description of changes

This is required when running idealized cases where CO2 levels can fall outside the expected limits.

Specific notes

Contributors other than yourself, if any:

CTSM Issues Fixed (include github issue #): #80

Are answers expected to change (and if so in what way)?
Answers should not change for normal use cases, but CLM will not stop due to unexpected CO2 levels.

Any User Interface Changes (namelist or namelist defaults changes)?
No. Potentially, could be introduced as an option with a keywork, e.g. warn_on_co2 to switch between casting an error or a warning.

Does this create a need to change or add documentation? Did you do so?

Testing performed, if any:
(List what testing you did to show your changes worked as expected)
(This can be manual testing or running of the different test suites)
(Documentation on system testing is here: https://github.com/ESCOMP/ctsm/wiki/System-Testing-Guide)
(aux_clm on derecho for intel/gnu and izumi for intel/gnu/nag/nvhpc is the standard for tags on master)

NOTE: Be sure to check your coding style against the standard
(https://github.com/ESCOMP/ctsm/wiki/CTSM-coding-guidelines) and review
the list of common problems to watch out for
(https://github.com/ESCOMP/CTSM/wiki/List-of-common-problems).

[TomasTorsvik]

@mvdebolskiy , @mvertens - I came back to this issue as I need to run more idealized emission scenarios, and it would be nice to avoid using sourcemods.