Choose one measurement per subject
select_meas(data, subject = "subject", prop = 1, verbose = F)
dataframe containing the sampled rows
dataframe as returned by prepare data
name of the column containing the subject identifier
optional - proportion of measurements to sample
if TRUE information about sample size is printed out
Mandy Vogel
function samples one measurement per subject, if prop < 1 additional
a prop*100 percent will be sampled from the measurements
the function is called inside do_iterations
and may not called directly