Update key for enrichment material modification in Lithium material class

[keckler]

Description

The material modification keyword for enrichment in the Lithium material class is ambiguous to the user as to which isotope is being enriched. This is particularly confusing for the case of lithium, because 'enriched lithium' means different things to different people. People who want to use lithium as an absorber typically think 'enriched' refers to Li6, while those who use lithium in coolant materials think 'enriched' refers to Li7.

In addition, the keyword is just inconsistent with all the other material classes which employ enrichment material modifications. In all other existing cases, the isotope is specified in the key.

This PR updates the key for the enrichment material modification in the Lithium class to avoid the ambiguity and be consistent with the other material classes.

Tests have been added.

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Checklist

• Tests have been added/updated to verify that the new or changed code works.
• Docstrings were included and updated as necessary
• The code is understandable and maintainable to people beyond the author
• There is no commented out code in this PR.

If user exposed functionality was added/changed:

• Documentation added/updated in the doc folder.
• New or updated dependencies have been added to setup.py.

[john-science]

I have the same feeling about this as I did about the Thorium Oxide PR: it seems good, I wonder why this wasn't necessary before.

I suppose the main concern is: do we think this will break anyone's downstream models?

Perhaps we can talk about that over some sort of a call today?

[john-science]

Nice.

Also, thanks again for the unit test coverage of the new code.