Add more Data Frame examples

[usmanmohammed]

Fixes #600

I think it could be beneficial to show more examples on how to manipulate data frames.

Union of two data frames

DataFrame unionDf = df.Union(df);
unionDf.Show();

+----+-------+
| age|   name|
+----+-------+
|null|Michael|
|  30|   Andy|
|  19| Justin|
|null|Michael|
|  30|   Andy|
|  19| Justin|
+----+-------+

Add new column

df.WithColumn("location", Lit("Seattle")).Show();

+----+-------+--------+
| age|   name|location|
+----+-------+--------+
|null|Michael| Seattle|
|  30|   Andy| Seattle|
|  19| Justin| Seattle|
+----+-------+--------+

Rename existing column

df.WithColumnRenamed("name", "fullname").Show();

+----+--------+
| age|fullname|
+----+--------+
|null| Michael|
|  30|    Andy|
|  19|  Justin|
+----+--------+

Filter null rows in age column

df.Filter(Col("age").IsNull()).Show();

+----+-------+
| age|   name|
+----+-------+
|null|Michael|
+----+-------+

Fill null values in age column with -1

df.Na().Fill(-1, new[] { "age" }).Show();

+---+-------+
|age|   name|
+---+-------+
| -1|Michael|
| 30|   Andy|
| 19| Justin|
+---+-------+

Drop age column

df.Drop(new[] { "age" }).Show();

+-------+
|   name|
+-------+
|Michael|
|   Andy|
| Justin|
+-------+

[imback82]

LGTM, thanks @usmanmohammed!