'**SUMMARY**
Returns a FeatureSet when sliced. Previously used to
return list. Now it is possible to use FeatureSet member
functions on sub-lists'

def __getitem__(self, key):

    if (type(key) is types.SliceType):
        return FeatureSet(list.__getitem__(self, key))
    else:
        return list.__getitem__(self, key)
