Explicitly close xr.Dataset in VarHandler.cmorize()

[tomvothecoder]

Description

By explicitly closing the multi-file xr.Dataset object, we avoid any possibility of resource locking with .nc files

This is an attempted fix for E3SM-Project/zppy#549.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• 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)