add interest rate, operating cost and maintenance cost to the OEO

[madbkr]

Description of the issue

As mentioned in #1968 interest rate, operating cost and maintenance cost are terms used in a definition in the new tasks module.
This may also relate to #1914.

Ideas of solution

interest rate is an economic value that shows how much interest is added to a value per time period.
operating cost is a fixed cost that is given due to operating a thing day-to-day.
maintenance cost is a fixed cost given by maintaining a thing so it stays operational.

Both operating and maintenance costs are covered by definition fixed cost which is in the OEO.
A fixed cost is a cost for operation and maintenance that does not vary with the amount of goods or services produced.
So I changed my original definition to reflect that.

Workflow checklist

• 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

[stap-m]

operating cost is a fixed cost that is given due to operating a thing day-to-day.
maintenance cost is a fixed cost given by maintaining a thing so it stays operational.

"thing" is not good wording for ontology definitions... 😉 Since we don't have any axioms from fixed cost to any "thing that causes the cost", I'd suggest to use examples from OEO:
operating cost is a fixed cost that is caused due to the day-to-day operation, e.g. of a manufacturing plant or power plant.
Same for maintenace cost.