Incorrect implementation of the ikine_6s method for the Puma 560

[Nikolay-Zagrebin]

When calculating with the tool, the inverse matrix of the tool is subtracted as if it were a base, it should be the other way around.

def ikine_6s(self, T, config, ikfunc):
....
if tool is not None:
T = tool.inv() * T

Must be T = T * tool.inv()