Revert the setting "mpiTasksPerNode" back to armi and rename it

[mbk-tp]

What is the change?

Putting the setting mpiTasksPerNode back into armi with a new name nTasksPerNode.

Why is the change being made?

In a prior PR, the setting mpiTasksPerNode was moved to an internal cluster plugin. However some codes still expect this setting to be in the ARMI framework.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.

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

• The release notes have been updated if necessary.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.

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2 out of 3 committers have signed the CLA.

✅ mbk-tp
✅ john-science
❌ Bharat Medasani

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