add OPEX to the OEO

[madbkr]

Description of the issue

As mentioned in #1968 OPEX is a term used in a definition in the new tasks module. Since this not a trivial concept I would like it to be added to the OEO.

Ideas of solution

I crafted a first idea for a definition:
Ongoing operational expenditures (OPEX) are costs for day-to-day operations (e.g. rent, wages) of a company not including whatever value is actually produced by said company.
OPEX would be an alternative label.

Workflow checklist

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

I am aware that

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