[TOREE-492] Add DataFrame displayer

[manuzhang]

[manuzhang]

@lresende could you please take a look ?

[lresende]

• What's the main difference between this and %dataframe
• Do we have to do any special handle for dataset ?
• Could you please add a little user documentation on how to use it (other displayers might be helpful as well).
• Could you please add a simple test case

[manuzhang]

@lresende

1. I was not aware of the %dataframe magic. This PR adds a syntactic sugar over df.head(20) where both df and df.head need %truncation off to be displayed in html format. In practice, I do a lot of df.show()s to get sample results which are not pretty printed. Now I know it can be achieved through %dataframe but I wonder it would better to display sampled df in html format by default (without any magic).

2. Do you know where to add the corresponding test case and doc for ScalaDisplayer ? All I've found are some notebook tutorials.

[mohammedi-haroune]

@manuzhang It would be great if the displayer works by default (the same way as a pandas data frame). Is that working for you ?

[manuzhang]

@mohammedi-haroune yes, that works for me

[mohammedi-haroune]

@manuzhang Thanks for you reactivity. I will check it out.