Extract the number of samples each feature was detected in for each experiment in a Qfeatures object
get_samples_present.RdWhen exploring the proteome coverage, it is instructive to consider how many samples a protein
was present in for each level of filtering. This function takes a Qfeatures object and tallies how many samples a variable in the rowData
was present in. When the row variables are the protein names, the tally is thus the number of samples the protein was present in.