API: deprecate combineAdd and combineMult methods?
On a DataFrame, we have the [`combineAdd`](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.combineAdd.html) and [`combineMult`](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.combineMult.html) methods:
- they are a redundant, as they are defined as `self.add(other, fill_value=0.)` and `self.mul(other, fill_value=1.)` (that is the actual implementation), which is also just cleaner to use IMHO.
- they have awkward (and non-pep8) names
- I assume they are not used a lot (I found 3 relevant references on StackOverflow for combineAdd ([SO1](http://stackoverflow.com/questions/28421137/subtract-two-dataframes-with-non-overlapping-indexes/28421255#28421255), [SO2](http://stackoverflow.com/questions/27744908/computing-sum-of-pandas-dataframes/27746836#27746836) and [SO2](http://stackoverflow.com/questions/20927701/pandas-numpy-how-to-turn-column-data-into-sparse-matrix/20929504#20929504), which is more than I expected), and none for combineMult

Therefore, I was thinking we could maybe deprecate these (as kind of clean-up)?
