Reprogen: Improve Datatype annotation.

[Aklakan]

The Datatype annotation can be used to map Java objects to RDF types, such as Java int to xsd:gYear. Shacl requires namespaces to be provided as literals of type xsd:anyUri. The following does not work, probably due to a missing type mapping:

  @Datatype("http://www.w3.org/2001/XMLSchema#anyUri")
  String setIri(String str);

[Aklakan]

In general, there should be support for lenient reads: if the existing data uses xsd:string instead of xsd:anyURI, the getIri method should still be capable of matching the datatype.

Perhaps the read methods should be annotated with all the datatypes they should be able to read, whereas the write method is annotated with the canonical datatype.

This would allow for implementing the "robustness principle": be liberal in what you accept and conservative in what you send.
In short: Lenient read / canonical (or strict) write.