purchase_cost calculation

[ChrisKahrs]

In the NewsVendor.py file... I don't think "excess_inventory" should be in the calculation for "purchase_cost". It is handled in the "holding_cost" section and it messes up the calculations for everything else in purchase costs for new items. Am I incorrect? I took it out and everything now seems to be working ok. I will do a fix and pr if you believe this is also correct. Thanks!

Line 84 (newsvendor.py)
excess_inventory = max(0, inv_on_hand - demand)
short_inventory = max(0, demand - inv_on_hand)
purchase_cost = excess_inventory * self.cost * order_qty *
self.gamma ** self.lead_time
holding_cost = excess_inventory * self.h
lost_sales_penalty = short_inventory * self.k

[osarwar]

Hey @ChrisKahrs, thanks for pointing this out! I'd love a PR if you can manage it.