Resolve "Remove usage of Dune::Function"

What does this MR do?

Updates the dynamic power local interpolation to assume that f is a callable instead of a Dune::Function object

Is there something that needs to be double checked?

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Can this MR be accepted?

• I've read contribution guide
• Implemented
• Added/Updated tests
• Added/Updated documentation
• Pipelines passing

• Delete branch option set
• Fast-forwaded git merge is possible
• Added entry to CHANGELOG.md

Related issues

Closes #47 (closed)