fractile: fix for nans ending up in output
fractile: fix for nans ending up in output

+ Size of the ensemble has to be taken from SortedValues which is used
  for calculation. Err on the side of extra cautious and push_back the
  guard band value to the end of the vector
+ Check for nans when calculating mean and standard deviation

fmidev/himan,fmidev/himan,fmidev/himan