normalize raw readings as percent of median/mean vehicle control wells, per assay, per plate. This function is called by normalize_per_plate, should not be called directly by user. #'
normalize_single_assay(dt, nctrl)data.frame contains essential columns with the raw data.
the name (spid) of the vehicle/solvent control used for calculation
data.frame with normalized value columns. 'nval_mean' column is the normalized value calculated using the mean of vehicle control wells, 'nval_median' column is the normalized value calculated using the median of vehicle control wells.