compare_namelists.py: allow comparing multiple namelist groups with same name

[jimmielin]

Description

This changes compare_namelists.py changes to support comparing multiple namelist groups of the same name. This namelist configuration is used in CAM-SIMA for separate history tapes but currently triggers a NLCOMP failure when multiple tapes are specified. Please refer to issue.

• Closes Namelist groups with the same group name will fail NLCOMP #4987

Checklist

• My code follows the style guidelines of this project (black formatting)
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that exercise my feature/fix and existing tests continue to pass
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding additions and changes to the documentation - not applicable unless there's somewhere documenting this behavior?