virtual track sum

[nettam]

if vtrack is defined with func="sum" , I would expect that if there are no values in the source of the vtrack in the given time.shift then the value 0 will be returned (and not NA).
Using this for example to count how many children a mother has previously had (need to support twins) - will never return 0

[aviezerl]

It is easy to implement, but don't we sometimes want to distinguish between no-data cases and actual 0? In the case of the children 'size' may be enough - and it does return 0 instead of NA. @nettam - what do you say?