Restructuring references and cross-references for infrastructure elements

In the forum entry [http://www.railml.org/forum/ro/?group=1&offset=0&thread=58&id=234] Susanne suggested to extend the concept of cross-referencing to other infrastructure elements. In preparation of a major release, it is necessary to review all references and cross-references in order to define a more generic concept, which limits redundancy, but provides intuitive usage for all applications.