Uses a Symbol's Dataframe, to build a new Dataframe,
        with the data converted to the new units
        
        Parameters
        ----------
        symbol : str or tuple of the form (Dataframe, str)
            String representing a symbol's name, or a dataframe
            with the data required to be converted.  If supplying a 
            dataframe, units must be passed.
        units : str, optional
            Specify the units to convert the symbol to, default to CAD 
        system : str, optional
            If None, the default system specified at instantiation
            is used.  System defines which conversion approach to take.
        tag : str, optional
            Tags define which set of conversion data is used.  If None, the
            default tag specified at instantiation is used.

def get_converted(self, symbol, units='CAD', system=None, tag=None):
        """
        Uses a Symbol's Dataframe, to build a new Dataframe,
        with the data converted to the new units
        
        Parameters
        ----------
        symbol : str or tuple of the form (Dataframe, str)
            String representing a symbol's name, or a dataframe
            with the data required to be converted.  If supplying a 
            dataframe, units must be passed.
        units : str, optional
            Specify the units to convert the symbol to, default to CAD 
        system : str, optional
            If None, the default system specified at instantiation
            is used.  System defines which conversion approach to take.
        tag : str, optional
            Tags define which set of conversion data is used.  If None, the
            default tag specified at instantiation is used.  
        """
        if isinstance(symbol, (str, unicode)):
            sym = self.get(symbol)
            df = sym.df
            curu = sym.units
            requ = units
        elif isinstance(symbol, tuple):
            df = symbol[0]
            curu = symbol[1]
            requ = units
        else:
            raise TypeError("Expected str or (DataFrame, str), found {}".format(type(symbol)))
        
        system = system or self.default_system
        tag = tag or self.default_tag
        
        conv = self.converters[system][tag]

        newdf = conv.convert(df, curu, requ)
        newdf = pd.merge(df, newdf, left_index=True, right_index=True)
        newdf = newdf[df.columns[0] + "_y"].to_frame()
        newdf.columns = df.columns
        return newdf