BUG: Ensure consistent ordering of amounts, prices & multipliers
BUG: Ensure consistent ordering of amounts, prices & multipliers

Previously, we have assumed that the `amounts` and `last_sale_prices`
lists have the same order as the `value_multipliers`. This is not
correct, since to populate the `amounts` and `last_sale_prices` lists
we iterate over a `dict` (self.positions). The order of this `dict`
can change in arbitrary ways when it is updated, which occurs when
we call `update_positions`. Our `value_multipliers` however are stored
in an `OrderedDict`, meaning the order of existing key/value pairs
is not changed when they are updated.

To address this issue, we make sure that `self.positions` subclasses
`OrderedDict`.

magne-max/zipline-ja,florentchandelier/zipline,bartosh/zipline,enigmampc/catalyst,humdings/zipline,alphaBenj/zipline,magne-max/zipline-ja,bartosh/zipline,quantopian/zipline,wilsonkichoi/zipline,quantopian/zipline,alphaBenj/zipline,Scapogo/zipline,Scapogo/zipline,wilsonkichoi/zipline,enigmampc/catalyst,umuzungu/zipline,umuzungu/zipline,humdings/zipline,florentchandelier/zipline