ExternalModel - when extractor output is not a single value is transformed to type "Any" in the samples dataframe

When one (or more) Extractor(s) of an ```ExternalModel``` produces a output that is not a single value (```Vector```, or ```DataFrame```, or ```Dict```,...) in the resulting ```DataFrame``` produced by the ```evaluate!``` function it is "promoted" to type ```Any```. This behaviour should be changed in order to keep the exact type of the produced output.  