warning in buildModels if trainControl indexes are not set

[zachmayer]

If buildModels is not passed a trainControl object with pre-definded indexes, for use with all models, we should raise a warning and try to set the resampling indexes ourselves, based on trainControl$method.

If we can't set the indexes, we should raise an error.

This probably requires a new function setIndexes.

[zachmayer]

Closed by #56