Removing Composite.setNumberDensities()

[john-science]

What is the change? Why is it being made?

Here I remove the method Composite.getNumberDensities() at the request of the user base. It leads to too many complicated concerns about number densities in, say, a Block which contains multiple Components with differing number densities. So now the method getNumberDensities() only exists on Components, which by definition have no children.

close #1374

SCR Information

Change Type: features

One-Sentence Description: Removing Composite.getNumberDensities to simplify concerns about auto-homogenizing materials.

One-line Impact on Requirements: NA

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Checklist

• [x] This PR has only one purpose or idea.
• [x] Tests have been added/updated to verify any new/changed code.
• [x] The documentation is still up-to-date in the doc folder.
• [x] The code style follows good practices.
• [x] The dependencies are still up-to-date in pyproject.toml.

[john-science]

@jakehader @drewj-tp Bump.

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