No class datatypes

[GeertThijs]

In § Attribute definition and usage. Like for the issue #x, there is no explanation. If the explanation would be that it makes a diagram more simple then that is not true: it simplifies most diagrams as there is no association line on the diagram. And the toolchain makes objectproperties of associations anyhow.

[costezki]

The reason for this is to keep a clear separation between (a) properties that take a literal value (governed by a datatype, and conceptually corresponding to owl:DatatypeProperty) and (b) properties that take a reference value (governed by a class, corresponding to owl:ObjectProperty). Doing so has two advantages: (a) it confers homogeneity to the model, which enhances readability and decreases ambiguity; (b) the toolchain implementation is easier as there is a clear treatment and interpretation.