Simplified indexing for ttest
Simplified indexing for ttest

- The ttest can only use one feature, so we can greatly simplify
indexing the feature selections, which speeds up the computation

kunallillaney/thunder,pearsonlab/thunder,poolio/thunder,oliverhuangchao/thunder,broxtronix/thunder,zhwa/thunder,jwittenbach/thunder,broxtronix/thunder,pearsonlab/thunder,thunder-project/thunder,j-friedrich/thunder,kcompher/thunder,j-friedrich/thunder,zhwa/thunder,mikarubi/thunder,kcompher/thunder,poolio/thunder,kunallillaney/thunder,mikarubi/thunder,oliverhuangchao/thunder