Finds a namespace with the given uri and prefix, in the scope of the hierarchy.
Searches from the current element and goes up the hiararchy until a match is found. If no match is found, returns null.
Either prefix or uri should be null. Results are undefined if both are specified.
@param uri The namespace to look for. If this is specified,
prefix should benull.
@param prefix The prefix to look for. If this is specified,
uri should be null.
@return Returns the matching namespace declaration, or
null if none was found.
@see #declareNamespace(String,String)
@see #declareNamespace(OMNamespace)
@see #getAllDeclaredNamespaces()