Update quantity repr

[jules-ch]

• Closes Quantity repr #1495

• 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

• Update CHANGES

• Add test for repr

• Update repr function to include magnitude repr & units

[andrewgsavage]

any reason this wasn't merged for the last releases?

[codspeed-hq]

CodSpeed Performance Report

Merging #1579 will not alter performance

_{Comparing jules-ch:quantity-repr (9c082fa) with master (b436902)}

Summary

✅ 437 untouched benchmarks

[jules-ch]

Gonna rebase and I think we can merge this.
@andrewgsavage @hgrecco

[jules-ch]

is there a reason why we are using only 6 decimals for float.
repr of float is already truncating things

[andrewgsavage]

not that I am aware of. what would be best for the repr? I'd just use the repr of the float