Access BEL annotation resource values

[abargnesi]

The BEL::Namespace module provides NamespaceDefinition instances for all published OpenBEL namespaces. This is not true for annotations.

This enhancement should provide access to published OpenBEL annotations. Each AnnotationDefinition should provide keyword, url, rdf_uri, and values. The values are fetched from the specified URL.

The ResourceIndex should return these AnnotationDefinition instances for annotations.