Add `useful energy`

[stap-m]

Description of the issue

We have primary, secondary and final energy consumption, and we have energy demand.
But we don't have useful energy (Nutzenergie). This is needed e.g. in the context of municipal heat panning, but is also a relevant concept for the energy domain in general.

We should discuss useful energy, either as consumption or demand, and add a respective quantity value.

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
• any ontology content that is proposed in this issue may be altered as needed to fit the structure and conventions of the OEO

[stap-m]

Notes from oeo-dev 130:

• IEA - Glossar: Energy available to end users to satisfy their need for energy services. As a result of transformation losses at the point of use, the amount of useful energy is lower than the corresponding final energy demand for most technologies. See also energy services.
• consumption vs. demand? depends on the context (past vs future)
• useful energy: realization of a function (main process), e.g. heat pump
• process attribute for the main process
• OEO:
  • Consumption is the process of using something and thereby reducing its amount.
  • Energy use is the consumption of an energy carrier making use of the energy it carries.
• MENO processes, produces and consumes energy betrachten, functions for technical processes