Support list type as series' element

[ariefrahmansyah]

Currently, gota only supports four basic types (bool, float, int, string) but not for list types ([]bool, []float, []int, []string).

Any plan to support list types?

Example:

	df := dataframe.New(
		series.New([]string{"b", "a"}, series.String, "COL.1"),
		series.New([]int{1, 2}, series.Int, "COL.2"),
		series.New([]float64{3.0, 4.0}, series.Float, "COL.3"),
		series.New([][]int{{1, 2}, {3, 4}}, series.IntList, "COL.4"),
	)
	fmt.Println(df)

	// Output:
	// [2x4] DataFrame
	//
	//     COL.1    COL.2 COL.3    COL.4
	//  0: b        1     3.000000 [1 2]
	//  1: a        2     4.000000 [3 4]
	//     <string> <int> <float>  <int_list>

It's related to #102