Pointing the Component.density method at 3D material density

[john-science]

Description

Presumably by accident, in the past the Component.density() method pointed at the 2D/linear expansion-only/pseudodensity, instead of the usual physical density of materials. This PR corrects that issue.

@opotowsky
NOTE: In testing, however, I found no materials in ARMI that explicitly defined separate pseduodensity() and density() method. So, using existing materials I could make a unit test that would fail after the change. But the moment I mocked up a material with clearly different density and pseudodensity, the test immediately failed.

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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.

• 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.

[opotowsky]

Wow, fascinating that this didn't break for any of the materials. Seems like...luck?

Anyway, this turned out to be a lot simpler than I thought!

[opotowsky]

Is this the only material you found that needed this update?

[john-science]

Well, I randomly happened across this one.

Perhaps we could look through all the materials for similar mis-namings.

[opotowsky]

This is fine to merge @john-science