Add metric: area under global precision-recall curve #588
Description
The metric is described here. So, the main difference from our map or precision is that it's not a query-wise metric, but a global one. In other words, there is the only precision-recall curve for all queries.
A few years ago I used an implementation from here, but not sure if it would be easy to adapt that code.
Check the existed OML metrics for reference.
