Add a loadXML function to load an sofa scene from xml in python.

[damienmarchal]

The added loadXML function permit to load old xml sofa scene from in python.

This is quite convenient to combine multiple of them or add extra features.

from splib3.loaders.xmlloader import loadXML

class Controller(Sofa.Core.Controller):
        def __init__(self, *args, **kwargs):
		self.target = kwargs.target)

def createScene(root):
	loadXML("MyScene.xml", root)

	root.addChild("Robot")
	loadXML("MyRobot.xml", root.Robot)

	root.Robot.addObject(Controller(target=root.Robot))

[EulalieCoevoet]

Nice! An example at the end of the file like you usually do would be great, otherwise good for me.

[damienmarchal]

@EulalieCoevoet as requested, I added an example.