Fix pixie plugin issues

[markcmiller86]

Description

This resolves some of the issues reported in some of the comments of #20982

• Fixes parallel decomposition logic to eliminate overlap between MPI ranks
• Ensures node-centered and zone-centered variables are defined correctly
• Replaces custom parallel decomp logic with utilities available in avtDatabase

Type of change

• Bug fix~~
• New feature~~
• Documentation update~~
• Other~~

How Has This Been Tested?

Reminders:

• Please follow the style guidelines of this project.
• Please perform a self-review of your code before submitting a PR and asking others to review it.
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Checklist:

• I have commented my code where applicable.~~
• I have updated the release notes.~~
• I have made corresponding changes to the documentation.~~
• I have added debugging support to my changes.~~
• I have added tests that prove my fix is effective or that my feature works.~~
• I have confirmed new and existing unit tests pass locally with my changes.~~
• I have added new baselines for any new tests to the repo.~~
• I have NOT made any changes to protocol or public interfaces in an RC branch.~~

[biagas]

Are there any regression tests that will be affected by this change?
Is there new sample data for creating a test, or can the problem being addressed be demonstrated by current sample data?

[biagas]

Do the new methods warrant method comment blocks?

[biagas]

@markcmiller86 looks like the scope of this PR has expanded beyond PIXIE specific fixes.
Should the notes/title be reworked, or should the HDF5 changes in other readers be put in a new PR ?