WIP: Fix Offset conversion with prefix & conversions using offset units

[jules-ch]

• Closes Problems when converting values with offset and prefixes #1504
• 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

Well it works but the implementation is so ugly.

It basically using an iterator to get converter from unit to base unit & making conversion from src to base & then from base to dst.

prefixed_src -> src_referenced -> ... -> base

base -> ... -> dst_reference -> prefixed_dst

It is similar to what happens with multiplicative, but this time I'm using an iterator instead of the recursive function used in get_roots_units with the accumulator.

I had to use some trickery for to identify the prefixed unit as non multiplicative.
I think we need to work on this, since if a converter is non multiplicative in this tree going back to base unit, well conversion will be non multiplicative.

If you guys have insights, I feel like this is clearly not OK in terms of clarity.

[OrangeChannel]

See #1381 (comment) . I also don't think prefixing offset units should be allowed at all. Practically I get that a kilodegC is 1000 °C but an object at 1 kilodegC is not a thousand times hotter than something that is 1 degC right? The multiplication implies by prefixes doesn't physically make sense here.