Bug fix for core mesh params + nonUniformAssems

[albeanth]

Description

A bug was found for the core mesh params referenceBlockAxialMesh and axialMesh when considering assemblies that have the nonUniformAssem flag.

These core params should not include nonUniformAssems. This PR resolves this.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify that the new/changed code works.
  • I think we should create an issue to make updating/setting core.p.referenceAxialMesh and core.p.axialMesh consistent and specifically testable.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes (location doc/release/0.X.rst) are up-to-date with any bug fixes or new features.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in setup.py.