Positions access did not cause update

[zero-element]

https://github.com/pmorissette/bt/blame/2c6bb3f7d1dd4aa253df406b1d71156188de2800/bt/core.py#L1252
should be

if self._needupdate or self.now != self.parent.now:

the same as the values and prices

[zero-element]

#438