Add wind power density to the OEO

[madbkr]

Description of the issue

Wind power density (WPD) is a term used in the definitions of the tasks module (see #1891). As decided in the dev meeting there should be a separate issue to discuss the definition.
It was also discussed if this should be the subclass of areal power density.

Ideas of solution

I propose this definition:
wind power density (WPD) is a quantity vale of the power wind has in a specific area. It is calculated as 0.5 * air density * (wind speed to the power of 3)

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