Add methods to parse a resource map and extract relationships

[gothub]

The methods createFromTriples() and serializeRDF() take the relationships contained in a DataPackage and serialize them into an RDF file (ORE resource map) that conforms to the DataONE package specification

Add methods to support the inverse operation of parsing an RDF resource map and extracting the package relationships so that they can be loaded into a DataPackage. These methods will be added:

• parseRDF()
  • parse an RDF file and load the relationships into a ResourceMap object
• getTriples()
  • extract the relationships from a ResourceMap and return a representation that can be loaded into a DataPackage (i.e. triples in a data.frame)
  • this operation will remove (as much as possible) DataONE packaging relationships that may have been added by createFromTriples(), which include, but not limited to:
  • http://www.openarchives.org/ore/terms/isAggregatedBy
  • http://www.openarchives.org/ore/terms/Aggregation
  • http://www.openarchives.org/ore/terms/aggregates
  • these relationships will be added back in when a package is re-serialized (i.e. for update/upload)
  • resource maps can be created by one of several sources (DataONE client library, ?) so care must be taken when removing any ORE packing relationships

[gothub]

Added in commit 8d526e514be2f27a4efefb3d761d6fc0b5ac2737. In addition to the above description of getTriples(), this method also demotes package object identifiers by removing any prefix to the subject or object of relationships that contain a package identifier, e.g. remove the DataONE 'resolve' URI.