Fix `cosp_histogram_driver.py` not writing mean variables to netCDF

[tomvothecoder]

Description

• Closes #781

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] My changes generate no new warnings
• [x] Any dependent changes have been merged and published in downstream modules

If applicable:

• [ ] New and existing unit tests pass with my changes (locally and CI/CD build)
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have noted that this is a breaking change for a major release (fix or feature that would cause existing functionality to not work as expected)