Draft to include styles for features

[situx]

Closes #64

This draft is loosely based on the GeoJSONCSS approach:
https://wiki.openstreetmap.org/wiki/Geojson_CSS

It introduces a class for Styles, and properties to connect these to SpatialObjects and literals to store style information.

Ideas:

• Conditions for styles could be enforced with SHACL rules
• Linking geo styles to geometry roles that @nicholascar is proposing in Cater for and define basic Geometry roles #241

[situx]

This draft is mainly complete, but an open question for me is still the definition and positioning of text.
Should it be a label from the feature?
How to signal that a label should be there and where it should be?

[nicholascar]

I like the idea of a GeoSPARQL-aligned Semantic Web handling of styles for features 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.