FIX disallow MultiIndex in Series constructor GH4187
fixes #4187 (at least the segfault part).

Constructing Series with a MultiIndex raises NotImplementedError, the option (to implement this) is to return `.to_series()` ?

anyone have an opinion on this (whether `pd.Series(a_multiindex)` should return `a_multiindex.to_series()`).

If anyone says "yes", I'll update.
 @hayd do it internally (e.g. don't use **new**) (you can just do a_multiindex.to_series() in **init**)
 I hate `__new__` so much...

so I can just more this same couple of lines into the init?
 sorry....leave it there for now....that's ok (when refactor series will fix it)
 @jreback so should I change it to m..to_series() or shall we merge as NotImplemented (should fix this before 0.12, noone wants a segfault...).
 just leave as not implemented
