Add +f modifier to grdgradient -Q to specify file

[maxrjones]

Description of proposed changes

This PR adds a new +f modifier to the grdgradient -Q option to allow the user to specify what file should be created or read.

Fixes #5852

Reminders

• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
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• If adding new functionality, add a detailed description to the documentation and/or an example.

[PaulWessel]

Looks good!