Vector sum gets wrong result

[Tbolp]

v1 = Vector(0, 0, 0)
v2 = Vector(1, 2, 3)
print(v1.sum(v2))
it should be (1, 2, 3), but now i get (1, 1, 1)

[barbaBus]

I have worked out a fix for this issue and any other potential issues generated by the same bug.
In sum function, as well as in all the others functions, it is wrong to use "self.vecotr.index(x)" to identify the position of the vector component to use in the sum because if you have more identical values it returns always the first occurrence.