ERT-649 - Added support for percentile calculations
ERT-649 - Basic plotting of percentiles
ERT-649 - Added support for turning off refcase and observations
ERT-649 - Added statistics plot to GEN_DATA
ERT-649 - Improved quality of statistics plot
Fixed minor error
Merge pull request #540 from jepebe/ERT-649

Ert 649 P10, P33, Median, P67 and P90 plot