Missing support for generic geometry components in STL format

[jnwalther]

As discussed previously with @joergbrech, currently only STEP files are supported for inclusion as generic geometry components, whereas, according to the CPACS docs, STL and IGES should also be supported.

For the STL implementation, from my experience, we should avoid the STLAPI_Reader (which I believe is being used for the general STL inport in TiGL) at all costs, because it creates an individual face for each triangle and is therefore painfully slow. Instead, the RWStl class provides a good alternative.

Here is a working example in Python of how to read a Poly_Triangulation using RWStl and add it to a TopoDS_Face: https://dev.opencascade.org/content/displaying-polytriangulations