New methods in addition to decimation in asfreq?
Hi all, 

I can't find an existing ticket for this, hopefully I didn't miss it. If I understand correctly, asfreq currently only does decimation to change the frequency of a timeseries. I would like to suggest that we make this point more clear in the docstring and offer additional methods to aggregate values such as mean for example, but we could think of others as well, and even a custom function. I am happy to take a stab at the implementation.

Best,
Jonathan
