IGV: XML does not save all node properties

When C2 sends graphs over the network to IGV, each <graph> is sent separately. The same applies if C2 saves graphs to XML.

To save space, graphs that are saved from IGV only contains the incremental difference for each graph. This saves a lot of space (usually ~10x). The logic happens in Printer.java -> `exportInputGraph(.., difference=true, ...)` Unfortunately, there is a bug in this logic: the properties of the nodes are not saved correctly.

difference_false.xml - a XML saved from C2
difference_true.xml - opened difference_false.xml in IGV and save as difference_true.xml.

E.g. The root in the last graph of difference_true.xml has way less properties than in difference_false.xml.