This function defines the root equation for the biphasic model, i.e. V(t) - suppression_threshold = 0.
biphasic_root(timevec, params, suppression_threshold)numeric vector of the times, t, at which V(t) should be calculated
named vector of all parameters needed to compute the biphasic model, V(t)
suppression threshold: measurements below this value will be assumed to represent viral suppression. Typically this would be the detection threshold of the assay. Default value is 20.