Get a subset of columns from the given table X.
        X       a Pandas dataframe; the table to select columns from
        cols    a string or list of strings representing the columns
                to select
        Returns a numpy array with the data from the selected columns

def _get_columns(self, X, cols):
        """
        Get a subset of columns from the given table X.
        X       a Pandas dataframe; the table to select columns from
        cols    a string or list of strings representing the columns
                to select
        Returns a numpy array with the data from the selected columns
        """
        if isinstance(X, DataSet):
            X = X[cols]

        return_vector = False
        if isinstance(cols, basestring):
            return_vector = True
            cols = [cols]

        if isinstance(X, list):
            X = [x[cols] for x in X]
            X = pd.DataFrame(X)

        if return_vector:
            t = X[cols[0]]
        else:
            t = X.as_matrix(cols)

        return t