Get base namespace from ontology

[dgarijo]

Ar2tool doesn't know the base namespace of the ontology being designed. However, according to LOV, ontologies should have the following metadata property:
http://purl.org/vocab/vann/preferredNamespaceUri
If the ontology has that property, the base namespace can be changed with its value.

[idafensp]

AR2Dtool now assumes "base:" as the default prefix string for the base namespace. If it finds any "vann:preferredNamespacePrefix" it then uses its value as prefix string.