QTC error checks for missing data

[PDuckworth]

@cdondrup I wanted to highlight that the QTC abstract class only checks if data exists in all timepoints, by using the list of objects from timepoint[0] only. here

But (I believe) it still returns a qsr value for object pairs at timepoint[0], even if one of the objects doesn't have that timepoint. I don't know how.

FYI @yianni