Description
      Gets all resources of a given purpose from an RDDL document. You have to call
	  XML_RDDL::parseRDDL()
	  first. The purpose of a resource is specified by the 'xlink:arcrole' attribute.
      The purpose of a resource link determines what the link will be used for. Frequently the purpose
      of a link can be determined from the nature of the referenced resource. For example the purpose
      of an XML Schema is typically schema validation, yet a schema may be comprised of a number of
      included modules and even when included modules are themselves an XML Schema, the purpose is as a module.
      You can find a list of well known purposes at
      http://www.rddl.org/purposes/.