Deprecate: `level` parameter for aggregations in DataFrame and Series
For `sum`, `prod`, `count`, etc. there is a parameter `level` which is internally rewritten into `groupby`. 

I propose that the `level` parameter for these aggregations be deprecated and instead recommend to users that they use the `groupby` syntax.

The functions that would be affected are:

* any
* all
* count
* sum
* prod
* max
* min
* mean
* median
* skew
* kurt
* sem
* var
* std
* mad

I believe this list is comprehensive.