ENH:  GH2578, allow ix and friends to partially set when the key is not contained
      in the object
API: iat/iloc will raise if enlargement is specified as its ambiguous

CLN: set_value in Series/Frame now go thru indexing routings in core/indexing.py
Merge pull request #4515 from jreback/partial_setting

ENH/API:  GH2578, allow ix and friends to partially set when the key is not contained in the object