Allow clean_missing_data to accept expected total trials for subject rejection

[TommasoGhilardi]

The clean_missing_data function provides a simple and effective way to reject subjects and trials based on missing data. Currently, subjects are rejected based on the percentage of good trials in the data. However, I believe it would be good to allow users to specify the expected number of total trials for the participants, which could then be used to calculate the percentage of good trials. By default, the function should continue to use the data as it does currently, calculating the percentage based on the available trials.

This would be especially useful in developmental research, where the number of trials often varies due to sessions being interrupted at different points. By passing the expected number of total trials, the function could be used more effectively to reject subjects based on a consistent criterion, independent of the variability in trial counts across participants.