[SofaPython] added node.getForceField(index)
[SofaPython] added node.getForceField(index)

(cherry picked from commit 3d3677e51a5418e105e3afa4cb2db850bdfc1b2b)

FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa