Ontology: unit needs a new definition

[sfluegel05]

Description of the issue

The current definition of unit is _A unit of measurement is a standardized quantity of a physical quality._
This doesn't work for currencies which are units as well (as discussed in oeo developer meeting 8.2).
Currencies are needed for prices, see #331.
Therefore we need a new definition that includes currencies.

Ideas of solution

Workflow checklist

• [x] I discussed the issue with someone else than me before working on a solution
• [x] I already read the latest version of the workflow for this repository
• [x] The goal of this ontology is clear to me

I am aware that

• [x] every entry in the ontology should have a definition
• [x] classes should arise from concepts rather than from words

[akleinau]

We just have to delete the "physical quality" part, right? That was the problem why money didn't work with it

[l-emele]

I wasn't part of the discussion, but isn't it enough to delete the word _physical_ as it is still related to a quality?

[jannahastings]

Alternatively, we could try _A standardised quantity of a quantifiable entity_ ?

[stap-m]

unit is imported btw. Is it ok to change it?

[jannahastings]

unit is imported btw. Is it ok to change it?

Yes, I think it is fine as long as we add a comment giving an explanation.

[stap-m]

Nice. Ready for implementation?