Add `discount rate`

[KaiSchnepf]

Description of the issue

discount rate needs to be added.

Ideas of solution

Investopedia: The discount rate is an economic value that refers to the interest rate used in discounted cash flow (DCF) analysis to determine the present value of future cash flows.

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

[l-emele]

Small extension: The discount rate is an economic value with the unit percent that refers to the interest rate used in discounted cash flow (DCF) analysis to determine the present value of future cash flows.