Bug 148
Bug 148

getData method of AccumulatorAverageCollapsing needs to return data if it has any, even if the data is the initial data or the result of a reset.  The only time it should return null is if the data objects themselves are null.

etomica/etomica,ajschult/etomica,ajschult/etomica,etomica/etomica,ajschult/etomica,etomica/etomica