Add a "score card" script/notebook #408
Description
Our inference results are multidimensional and have a structure that makes analysing them in bulk a non-straightforward task. Calculating metrics on backtest results is also something we need to do on a regular basis. We've decided that it would be useful to have a "score card" script or notebook in PVNet that fits seamlessly with backtest results and contains functions for some basic metrics, to serve both as a tool and a jumping-off point for anyone who needs to create a more complicated evaluation function.