Out of sample WAR

The parameters involved in calculating openWAR should be able to be calculated with one set of data, and then applied to another set of possibly different data. For instance, we calculate parameters using data from 2000-2012 and then calculate players openWAR for the year 2013 based on the previous years.

No files, tests, or entry points are named. First identify where openWAR parameters are calculated and how player-year data is passed through the package; then define how parameters trained on 2000–2012 can be applied to 2013 data and what tests would demonstrate that separation.

Written by the indexing model from the issue text.