Truncate DataFrames centrally, rather than at one end

[takluyver]

This came up in discussion on #5550 and #5594. The default plain & HTML repr now truncates DataFrames to the first max_rows rows and max_columns columns. At least for rows, truncating in the middle (so we show the first max_rows//2 and the last max_rows//2 rows) would be more consistent with the display of Series and numpy arrays.

This isn't a major change, but it's not completely trivial either, which is why I haven't done it already. I'll try to get round to it, but if someone else wants to work on it, go for it.

[jreback]

@takluyver I think this would be real nice for 0.14!

I would just default this (though could have an option display.show or something , and make head_tail the default and allow head as well (I think their is an option for how MUCH to show already somewhere)....

[jreback]

@takluyver can you do this?

[jreback]

@takluyver ?

[bjonen]

I can submit a PR for this on the weekend, as it affects the display options similar to #6890

[jreback]

@bjonen excellent!

[takluyver]

Oh, sorry, the 'pandas' label is below the fold on GMail, so I didn't notice I was getting pinged. I'm happy for @bjonen to do it.