Spec for order-linked stops

[barnabee]

Write a spec for stop orders that are linked to another order, i.e.:

• as the order trades, the linked stop order is created/updated as needed with the size that traded
• if the order is cancelled, the created stop order remains at its size at time of cancellation

Open question: what happens to the stop's size if some of the position is closed?
- some kind of FIFO or LIFO updating of size
- size is min of created size and open position size
- something else?

Needs input: how important is this to users? Currently stop can be linked to all of a fraction of the position size but not to an order, so if you add a stop on the deal ticket it's created even if the position doesn't trade.