diff(dfToDiff, columnNames) {
        return this._join(dfToDiff, columnNames, ["out", "out"]);
    }
Find the differences between two DataFrames (reverse of join).
@param {DataFrame} dfToDiff The DataFrame to diff.
@param {String | Array} columnNames The selected columns for the diff.
@returns {DataFrame} The differences DataFrame.
@example
df2.diff(df2, 'id')