support for 'avg_zero' aggregation

Hi!
There is support for average aggregation which treats null values as zero, since for avg traffic/bandwidth
values, ignoring 'null' values makes hourly/daily traffic calculations wrong (hitcount(metric, "1d", true) for example).
Whisper is moving to a slightly more modern 2.6-ish style. Please change this to a list comprehension:

    values = [x or 0 for x in neighborValues]

Besides being more pythonic, it is less code. All around win :)