Add combined cycle gas power plant

[f-innatech]

Description of the issue

The differentiation between combined-cycle and open-cycle gas power plants is currently missing.
Since there is a 15-20% difference in efficiency this should be added.

Ideas of solution

Two Options:
either add open cycle and combined-cycle gas power plants below gas power plant
or
add only combined-cycle gas power plant as a subtype since a combined cycle plant contains the same parts as an open cycle but just with an extra cycle and a heat exchanger.

I think option two is more elegant.
Definition for combined-cycle gas power plant:
A combined-cycle gas power plant is a gas power plant that additionally has a heat exchanger as parts which is linked to a steam power unit.

Additional Axioms:
has some parts heat exchanger heat exchanger
has some parts steam power unit steam power unit

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

[f-innatech]

seee #1256