Allow `ud_convert()` to work with class "difftime" (fix for #3012)

[Aariq]

Description

Fixes #3012 . Checks if argument x is difftime, and if it is converts it to a units object with as_units(). Units are inferred from the class, so argument u1 isn't actually used in the conversion in this case. UI is a little weird here, but seems like this is mostly an internal function so probably better than adding argument defaults and allowing u1 to be NULL or something like that.

Motivation and Context

Review Time Estimate

• [ ] Immediately
• [ ] Within one week
• [x] When possible

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My change requires a change to the documentation.
• [x] My name is in the list of CITATION.cff
• [x] I have updated the CHANGELOG.md.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[Aariq]

Looks great- just make sure to update the change log!

I can do this, but I'll just point out that the changelog already includes a bullet about this function being re-factored for this version. This bug was an unforseen side-effect of that refactoring