Add UNSET directive to ParmParse

[Copilot]

Summary

Adds an UNSET directive to ParmParse that removes a previously-defined keyword from the parse table. This enables input files to conditionally suppress parameters — for example, an included file can cancel a key set by its parent.

amr.keyword = 5
UNSET = amr.keyword
# equivalent to never having set amr.keyword

Multiple keys can be removed in a single directive:

UNSET = key1 key2 key3

Additional background

Mirrors the design of the existing FILE directive. The new ParmParse::UnsetKeyword = "UNSET" static is exposed in the header for symmetry with FileKeyword. The directive is handled in addDefn() by erasing each listed key from the table; keys not present are silently ignored.

Tests added to Tests/ParmParse/inputs and Tests/ParmParse/main.cpp cover single-key unset, multi-key unset, and verifying that non-listed keys are unaffected.

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate