Numeric strings produce odd behavior in Feature Extractor regressor

[bomanimc]

The Feature Extractor Regressor handles numeric strings in a confusing way. Even though one might expect a numeric string input to the regressor to create predictions within the typical 0-1 range, it looks like string numbers will operate as labels that the regressor treats differently. It might be worth changing numeric strings into numbers if possible internally in the feature extractor class.

Some discussion about this on #1086.