if rankThenAggregate is used, na.treat does not have to be set in as.challenge, missing values are implicitly ranked last then. this is given as message now in as.challenge. However, if a report is compiled, significanceMap() needs specification of na.treat.

we could

• force to specify na.treat when doing report (i.e. sanity check in report())
• automatically set na.treat to infinity or -infinity depending on smallBetter (or function(x) max(x) +1 or function(x) min(x) -1 depending on smallBetter)