System IDs are not held in nodes in the tree, because they are usually the same
for a whole document.
This class provides a map from element sequence numbers to System IDs: it is
linked to the root node of the tree.
Note that the System ID is not necessarily the same as the Base URI. The System ID relates
to the external entity in which a node was physically located; this provides a default for
the Base URI, but this may be modified by specifying an xml:base attribute