Feature request: Check for possibly missing Cmax/Tmax and Tlast

[billdenney]

Find missing samples

1. Check if a subject has all nominal time samples which are not missing (requires that missing samples are provided with NA values)

Cmax/Tmax

1. Check the Tmax range, and
   1. If a subject is missing any samples in the Tmax range, warn that Cmax/Tmax may be missing
   2. If a subject is missing all samples in the Tmax range, add an exclusion rule for those samples

AUClast/Tlast

1. Check if Tlast is at the last nominal time possibly within a treatment group
   1. If Tlast is at the last nominal time, Tlast is not missing
   2. If Tlast is not at the last nominal time, and no samples are missing after Tlast, Tlast is not missing
   3. If Tlast is not at the last nominal time and samples are missing between Tlast and the first BLQ measure, Tlast may be missing; warn the user