Properly support trailing comma in char namelist entries in atm_in_paramgen

[jimmielin]

Tag name (required for release branches):
Originator(s): @jimmielin

Description (include the issue title, and the keyword ['closes', 'fixes', 'resolves'] followed by the issue number):

• Fixes Trailing comma at end of character namelist entry will throw error #294 (details in the issue)

Describe any changes made to build system:
atm_in_paramgen updated to support this. Doctests added to cover this case.

Describe any changes made to the namelist: N/A

List any changes to the defaults for the input datasets (e.g. boundary datasets): N/A

List all files eliminated and why: N/A

List all files added and what they do: N/A

List all existing files that have been modified, and describe the changes:
(Helpful git command: git diff --name-status development...<your_branch_name>)
M cime_config/atm_in_paramgen.py

If there are new failures (compared to the test/existing-test-failures.txt file),
have them OK'd by the gatekeeper, note them here, and add them to the file.
If there are baseline differences, include the test and the reason for the
diff. What is the nature of the change? Roundoff?

derecho/intel/aux_sima:

derecho/gnu/aux_sima:

If this changes climate describe any run(s) done to evaluate the new
climate in enough detail that it(they) could be reproduced:

CAM-SIMA date used for the baseline comparison tests if different than latest:

[nusbaume]

Thanks @jimmielin! I just had two small change requests.

[jimmielin]

Thanks @nusbaume! I've committed in your suggested changes, this should be ready once the tests finish run and pass.