URL catalog

With an URL catalog, you can substitute URLs by other URLs. This may be necessary if you mirror a repository. For example, the COLORE repository contains a lot of references (ontology imports) of the form http://colore.oor.net/path-to-some-onotology. Now you want that these references to point to ontohub. Therefore, you add an URL map from http://colore.oor.net to http://ontohub.org/colore