Draft for inclusion of surface textures and materials

[situx]

This pull request proposes to include textures and materials by examining related work such as CityGML/CityJSON, among others.
Finishing this pull request will close #438

The following contents are defined:

• Classes and properties for Materials and Textures
• Connection of surfaces to textures and materials (This part depends on the definition of 3D geometries in GeoSPARQL, which will be a separate pull request)

TODO:

• Evaluating whether literals need to be defined to store textures in the knowledge graph
• Cross-checking with the pull request for geometry styles in 2D Draft to include styles for features #418

[nicholascar]

I like the idea of a GeoSPARQL-aligned Semantic Web handling of textures and materials but I don't think it's core to GeoSPARQL so shouldn't be included in GeoSPARQL itself but rather an extension ontology (that could be published by OGC). This would be similar to the Fuzzy Geometries model: interesting but not core.

[situx]

@nicholascar agreed, it should not be core.
I think either, we might used the appearance module of CityRDF or we define an appearance vocabulary ourselves.
Given that we also need a vocabulary for 3D primitives, I think the fate of this pull request might depend on our discussion of #611