Function custom XML nodes

[PaulTalbot-INL]

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Pull Request Description

What issue does this change request address?

Closes #340

What are the significant changes in functionality due to this change request?

Uses RAVEN's new input spec feature (idaholab/raven#2332) to implement arbitrary function additional XML, which will be passed to the function as part of the meta object.

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For Change Control Board: Change Request Review

The following review must be completed by an authorized member of the Change Control Board.

• 1. Review all computer code.
• 2. If any changes occur to the input syntax, there must be an accompanying change to the user manual and xsd schema. If the input syntax change deprecates existing input files, a conversion script needs to be added (see Conversion Scripts).
• 3. Make sure the Python code and commenting standards are respected (camelBack, etc.) - See on the wiki for details.
• 4. Automated Tests should pass.
• 5. If significant functionality is added, there must be tests added to check this. Tests should cover all possible options. Multiple short tests are preferred over one large tes.
• 6. If the change modifies or adds a requirement or a requirement based test case, the Change Control Board's Chair or designee also needs to approve the change. The requirements and the requirements test shall be in sync.
• 7. The merge request must reference an issue. If the issue is closed, the issue close checklist shall be done.
• 8. If an analytic test is changed/added, the the analytic documentation must be updated/added.
• 9. If any test used as a basis for documentation examples have been changed, the associated documentation must be reviewed and assured the text matches the example.

[PaulTalbot-INL]

Requires idaholab/raven#2332.

[GabrielSoto-INL]

Changes LGTM