Returns the coefficient of variance of a column
        in percentage

def cvar_(self, col):
        """
        Returns the coefficient of variance of a column
        in percentage
        """
        try:
            v = (np.std(self.df[col]) / np.mean(self.df[col])) * 100
            return v
        except Exception as e:
            self.err(e,
                     "Can not calculate coefficient of variance")