create calc_targeted_close_short

[dpaiton]

No description provided.

[wakamex]

This may not require a new function from scratch. If the market impact of closing a short is the same as opening a long, then we may be able to piggyback on calc_targeted_long to tell us how much of a short we can close. This is currently the logic used in lpandarb.py, so I'm pretty sure it's the case, otherwise the LP and Arb bot wouldn't work.