Partial order handling

[multidis]

Deal in orderhandling! with a case when e.g. a limit-order is only partially filled.

[multidis]

7469952 allows to keep filling position up to the target level in the strategy targeting functions. Check however if that is appropriate: if an exchange/broker partially fills a limit order, the remaining part may be pending just with a different order id (?). Needs further exploration.