[fill_missing_correlation] and [check_filling_error] Take into account standard deviation of correlation

[cdemulde]

When calculating reliabilities, it would be good to take into account the standard deviation on data points filled in based on a correlation. A correlation calculations comes with a standard deviation. It is of little use to take this into account during the actual filling, but it does add some information on how reliable the filling is. Use that information to feed it back to the user.