Add addLink method in Binding_Base to create links dynamically

[marques-bruno]

This PR lets you write stuff like that:

import Sofa.Core
import SofaRuntime

SofaRuntime.init()
SofaRuntime.importPlugin("SofaComponentAll")

n = Sofa.Core.Node("root")
n.addObject('MeshTopology', name="topo")
n.addChild("node")

# test link creation on Base
n.node.addLink(name="aName", value=n.topo, help="Some help string describing the link's purpose")

# Test connection of "generic links", i.e. SingleLinks templated over abstract "Base" type, to specialized template links.
n.node.addObject("MechanicalObject", name="mo", topology=n.node.findLink('aName').getLinkedPath())

print(n.node.mo.topology.getName()) # => topo

requires a small change in sofa-framework/sofa (PR #1436 )

[damienmarchal]

ok for me. To merge after the corresponding pr in Sofa.