[vector] is a vector whose entries are the viability values observed in the presence of the
drug concentrations whose logarithms are in the corresponding entries of the log_conc, expressed as percentages
of viability in the absence of any drug.
trunc
[logical], if true, causes viability data to be truncated to lie between 0 and 1 before
curve-fitting is performed.
verbose
[logical] should diagnostic messages be printed? (default=FALSE)
Value
An estimate of the IC50 for the concentrations and viabilities provided