API: return_type argument in StringMethods.split()

[jorisvandenbossche]

xref #9667 Now we also have string methods available on the index, the possible options for return_type ('series', 'frame') are a bit confusing I think: it will be confusing for users to get a Index back even if he/she supplies return_type='series' on a Index.str.split, or to get a series back with Series.str.split(.., return_type='index')?

Possible ways to make this a better API:

1. An expand keyword (or another name), that indicates for False: give same dimension back (so for series/index keep it a series/index), and for True: expand series to dataframe.
   This would then be a duplicate for return_type of course. But the return_type was only introduced in 0.15.1 (and for partition it is still in a PR), so if we want to change this: better now than later. Or has this ship sailed?

2. easier solution of @jreback: return_type='same'|'expand' to satisfy this need? (and can be easily back-compat) -> so no need to change the name of the keyword, only the arguments.

[jreback]

if someone wants to tackle this in next day ok, otherwise can deprecate in 0.17.0 (its a simple deprecation so no big deal, can accept both)

[jorisvandenbossche]

IMO it would be really stupid do not do it kwow (giving users one release extra to get used to the to be deprecated keyword).
I can do it later this evening (or otherwise tomorrow certainly)