Please review PR's to fix Holt Winters bugs

This library's implementation of the Holt-Winters forecasting algorithm is the best one I could find online for golang, because it supports confidence intervals. However, it has two critical bugs for which I've provided fixes. Please review these two PR's that I just raised: 
https://github.com/rocketlaunchr/dataframe-go/pull/73
https://github.com/rocketlaunchr/dataframe-go/pull/74


