using ignoreImputeZeros was unclear

[asizemore]

I went to pull a collection with and without imputing zeros, but how to do that puzzled me for a bit because I didn't know which way ignoreImputeZeros was going to go. Was it going to ignore what the collection said and impute zeros anyway, or the opposite? The code tells all of course, but i think we could improve the naming to make it so that users dont have to go to the code. How about imputeZerosOverride? Then true imputes zeros and false doesnt, regardless of what the collection says.