Fix calls to computeCoef

I think this fixes #12 in `SampleSplitSuperLearner`, `SuperLearner`, `mcSuperLearner`, and `snowSuperLearner`. At least it seemed to be necessary and work for both  `SampleSplitSuperLearner` and `SuperLearner`. (Note that I have not actually tested `mc-` or `snowSuperLearner`.)

I'm skeptical though, because it looks like the call to `computeCoef` has been the way it is for 3 years. So if this is really the cause of #12, I'm surprised no one else has reported it.

I also had to make a couple extra changes for `SampleSplitSuperLearner` because of how the CV works there.

Please let me know if there's something weird about these changes and I'll keep looking for the issue.