BUG: Fix Grouper with multi-level index and frequency (GH7885)
close #7885 

need to have a test (you can put in test in the issue almost directly)
 test for level name and by level number
 write unit tests for both using the level number and name

then u will know of the code works and/or needs modification to handle level numbers/names. (I think it does)
 Thanks for clarifying. Will do.
 you specify levels like `level=1` (e.g. an integer)
 `ilevel_0` is ONLY for `.query` and doesn't apply here
 I get what you are saying. It's just that I see, as it states in the docs, that it is possible to construct a MultiIndex that has level names set to None. In this case, unless I am misunderstanding things, an integer index name will not work.
 of course, an integer when the names are `None` is a simply the level. Can you clarify what you mean?
 My mistake. You're right.
 ok gr8, was puzzled because your tests passed (which should not have)
 Okay. How does that look?
 I have applied your suggestions. 
 @jmorris0x0 thanks, merged in.

next time it IS easier!
