Fix redundant _FillValue attribute for MPAS fields

[xylar]

Previously, the write_netcdf() function was trying to add a _FillValue attribute to variables even if they already had one. With this change, we skip variables that already have a fill value defined

Description

• Closes #<ISSUE_NUMBER_HERE>

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)

[xylar]

@chengzhuzhang and @TonyB9000, I'm not really in a good position to test this. Can one of you give it a try on the data that was failing for @TonyB9000 before? I'm happy to iterate if needed.

[xylar]

I'm not sure if this addresses #255 completely or if there are multiple bugs there. I left it out of the description above for now.

[xylar]

It looks like CI is broken and it's nothing to do with my PR?

[TonyB9000]

@xylar I approve. The change looks perfectly harmless to me. I will run tests to see if this served to address the fault.

[chengzhuzhang]

@TonyB9000 have you confirmed that Xylar's fix has resolved this issue?

[TonyB9000]

@chengzhuzhang Yes - all of the v2_1 Omon (20+) that were failing were able to complete successfully, I should have followed up here - got distracted with other issues. Of course, why this would be an issue (adding a _fillValue to variables that already have one) is a mystery, but checking first and avoiding the "add" solved the problem.

[chengzhuzhang]

@chengzhuzhang Yes - all of the v2_1 Omon (20+) that were failing were able to complete successfully, I should have followed up here - got distracted with other issues. Of course, why this would be an issue (adding a _fillValue to variables that already have one) is a mystery, but checking first and avoiding the "add" solved the problem.

@TonyB9000 thank you for confirming. And thank you @xylar for the fix!

[TonyB9000]

@chengzhuzhang @xylar This issue (#259) and issue (#255) appear related and fixed. However, #255 also overlapped with the "lock=True/lock=False" issue, which (I believe) was resolved by completely eliding the inclusion of "lock=" in the command where it had appeared. Should we close these issues?