Normalise optical density against blank wells

Used by process_plate function for OD normalisation. Remains virtually unchanged from flopr::od_norm.

od_norm(pr_data, blank_well, od_name)

Arguments

pr_data	a long data.frame containing your plate reader data
blank_well	the well coordinates of one or more media blanks
od_name	the column name for the optical density data

Value

an updated data.frame with an additional column "normalised_OD"