Add min/max and fmin/fmax function to the interpretor registry

Issue/feature addressed

min/max (or fmin/fmax) function can be useful to define initial conditions and/or exact solutions. Currently those mathematical functions are not supported in the interpreter.

Proposed solution

Add min/max (and fmim/fmax) to the interpetor registry.

Implementation

A more detailed description of the changes. Please include any details necessary for reviewers to understand your work, and point out particular points would like them to pay particular attention to.

Tests

Suggested reviewers

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Notes

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Checklist

• Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• [ ] License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).