Two new classes: SvgWheelManager adds features towards eventually plotting datawheels over labelled entities (Chart Maps). For now, limited abilities to plot barycentre of entities and srround them with a rectangle. SvgTreeManager adds various features to robustly handle nested transformations in SVGs.

Also provided are test cases for both classes. testSvgWheelManager.py calls the most important features
of SvgTreeManager. Example of usage:

./testSvgWheelManager.py ../maps/Blank_Map_Africa_1932.svg > test.svg

CAUTION: does not yet support relative coordinates; you should convert your SVG to absolute coordinates using
Inkscape before using ./testSvgWheelManager.py , lest the first rectangle and barycenters are quite wrong
(others might work quite nicely).