add needed services in NsdResourceSet

There are dependencies between namespaces (NSD files). Each NSD file is loaded in a resource, the set of them is in a ResourceSet, and we need, as far as I understand, to iterate over some elements which may be in several resources. For example, main LNClasses are defined in the core IEC61850-7-4 NSD, but it is possible to have some other definitions in other NSD files. The class NsdResourceSet will provide the needed services