Gets all resources of a given nature from an RDDL document. You have to call
XML_RDDL::parseRDDL()
first. The nature of a resource is specified by the 'xlink:role' attribute.
A nature can be a stylesheet, a DTD, a HTML document, etc. You can find a list
of well known natures at http://www.rddl.org/natures/.
Parameter
string $nature - nature of the resources you want to get.
Return value
array array containing all resources with the specified nature.