delta units for std, var

[andrewgsavage]

• Closes Units for .var, .std methods on offset units. #1640
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

[codspeed-hq]

CodSpeed Performance Report

Merging #1992 will not alter performance

_{Comparing andrewgsavage:offset (cf6c276) with master (9972e60)}

Summary

✅ 437 untouched benchmarks