Enhance annotation and/or processor with default value support

[Aklakan]

Allow to specify a default value for the case that no triples exist for a property.
E.g. the absence of certain triples may indicate a value of 'true' for a property.
Most likely, we can reuse some existing annotation for that purpose.

interface Foo {
   @Iri("eg:bar")
   @Default(true) // Something along these lines
   Boolean isEnabled();
}

[Aklakan]

There is a @DefaultValue annotation for exposing Java functions as SPARQL functions:

	@IriNs(GeoSPARQL_URI.GEOF_URI)
	public static Geometry simplifyDp(
			Geometry geom,
			@DefaultValue("0") double tolerance,
			@DefaultValue("true") boolean ensureValid) {
		DouglasPeuckerSimplifier simplifier = new DouglasPeuckerSimplifier(geom);
		simplifier.setDistanceTolerance(tolerance);
		simplifier.setEnsureValid(ensureValid);
		Geometry result = simplifier.getResultGeometry();
		return result;
	}

This annotation could be reused for the purpose of the issue.

There are however different possible semantics for which it should be possible to specify it unambiguously:

• On get-access convert null to the default value (do not write to the model)

• On get-access write the default value into the model if the result would be null (i.e. get with a write side-effect)

• On get-access write null into the model if the result would otherwise be the default value

• On set replace the default value with null

• On set replace null with the default value

• On set just set the given value