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).
I don't like this, or that you updated the tests to pass in `[]`. Would you mind making `neighborValues` an optional kwarg and default to `[]` if it is set to None please? Something more like:

    def aggregate(aggregationMethod, knownValues, neighborValues=None):
        neighborValues = neighborValues if neighborValues is not None else []

Then please change the non avg_zero aggregate tests like they were previously.