API: nlargest/nsmallest, take_last -> keep

[jreback]

nlargest/nsmallest on frame should have take_last -> keep, in-line with #10236

here

no deprecation is necessary as this is new in 0.17.0

cc @sinhrks
cc @cpcloud

[sinhrks]

Do we need an option to keep all elements in this case, keep='all' or keep=True?

And Series.nlargest/nsmallest need deprecation.

[jreback]

you don't need to keep all option (that would be just sort :)

so just a direct keyword replacement for Series