API: add `inplace` kwarg to NDFrame.droplevel

[vbarda]

df.droplevel(level, axis=0)

vs

df.droplevel(level, axis=0, inplace=False)

Problem description

Currently .droplevel DataFrame / Series does not expose inplace kwarg, and it defaults to False under the hood, in NDFrame.set_axis method

cc @jreback @TomAugspurger

[gfyoung]

@vbarda : Go for it!

[TomAugspurger]

I think there's still a bit of discussion to be had on whether we *want* to add an inplace argument in new places. Personally, I'm not sure.

…

On Mon, Jul 23, 2018 at 12:02 AM gfyoung ***@***.***> wrote: @vbarda <https://github.com/vbarda> : Go for it! — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#21990 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABQHIoMHjnYHRdF6kuV4-M7XqCmeFReAks5uJVjUgaJpZM4VYDyl> .

[lithomas1]

This can be done inplace with Copy-on-Write, so closing.
(you would do something like df = df.droplevel(...)