added support for .stl export

[duswie]

added support for exporting to .stl files.
triangulate_face(..) was slightly changed to return the face normal. Tests are included.

Is it planned to move all export functions to convert.py?
I have thought about combining export stl and obj in the future and moving them there.

[balazsdukai]

First of all, welcome @Dadley and thank you for your contribution to cjio! Your very first, I think 🥳
The PR technically looks good, I tested the exporter a bit and it works fine.

Indeed, the plan is to move out all the exporters from the cityjson module to convert and make them functions instead methods. This means that ideally the stl exporter would be in convert too, similar to the glb or b3dm converters. This architecture will make it easier to eventually separate the various exporters into a standalone package to make it easier to maintain and to contribute to it.
However, I left the stl exporter as it is for now and I'll merge the PR since its been long open.

Please feel free to move the stl export to convert in separate PR, if you feel like doing it.
Re combining the stl and obj exporter. I'm not sure exactly what do you mean, so if you open another PR with a suggestion that would help the discussion.