This function returns an XML representation of the MaxTree.
As an example, the signal
[1, 2, 3, 4, 4, 4, 3, 2, 3, 5, 6, 6, 4, 4, 4, 3, 3, 4, 1, 2, 2]
is represented by the MaxTree:
<?xml version="1.0" ?>
<maxtree>
<node area="21" index="0" level="1" sx="210" sy="0" xmax="20" xmin="0" ymax="0" ymin="0">
<node area="17" index="0" level="2" sx="153" sy="0" xmax="17" xmin="1" ymax="0" ymin="0">
<node area="5" index="0" level="3" sx="20" sy="0" xmax="6" xmin="2" ymax="0" ymin="0">
<node area="3" index="0" level="4" sx="12" sy="0" xmax="5" xmin="3" ymax="0" ymin="0"/>
</node>
<node area="10" index="1" level="3" sx="125" sy="0" xmax="17" xmin="8" ymax="0" ymin="0">
<node area="6" index="1" level="4" sx="69" sy="0" xmax="14" xmin="9" ymax="0" ymin="0">
<node area="3" index="0" level="5" sx="30" sy="0" xmax="11" xmin="9" ymax="0" ymin="0">
<node area="2" index="0" level="6" sx="21" sy="0" xmax="11" xmin="10" ymax="0" ymin="0"/>
</node>
</node>
<node area="1" index="2" level="4" sx="17" sy="0" xmax="17" xmin="17" ymax="0" ymin="0"/>
</node>
</node>
<node area="2" index="1" level="2" sx="39" sy="0" xmax="20" xmin="19" ymax="0" ymin="0"/>
</node>
</maxtree>