Returns a Dataswim instance with a subset data based in positions
        Select a subset of the main dataframe based on position:
        ex: ds.subset(0,10) or ds.subset(10) is equivalent: it starts
        at the first row if only one argument is provided

def subset_(self, *args):
        """
        Returns a Dataswim instance with a subset data based in positions
        Select a subset of the main dataframe based on position:
        ex: ds.subset(0,10) or ds.subset(10) is equivalent: it starts
        at the first row if only one argument is provided
        """
        df = self._subset(*args)
        if df is None:
            self.err("Can not get subset of data")
            return
        return self._duplicate_(df)