Add simple option to create possible retrofit combinations for given archetyp

[FWuellhorst]

To study the impact of single retrofit measures (e.g. rooftop etc.) on the building envelope and energy system, studying possible combinations is often done.
This feature adds a simple option to generate buildings to project.py.

[PGorzalka]

@FWuellhorst have you already worked on this? My work on #679 includes the possibility to load specific elements by code, that might come handy for this task. You might want to have a look at the code in my fork.
I'm currently doing the final tests before opening the pull request here. The OpenModelica compatibility regarding AixLib issue 1080 that I contacted you about a few weeks ago is already solved.

[FWuellhorst]

@PGorzalka : Yes, its already finished, the PR is currently under review: #775
Nice to hear :)

[PGorzalka]

Thanks, that's good to see. I'll have a look and align my contributions with the way it's coded there.