triphasic_root: Triphasic root function
In SineadMorris/ushr: Understanding Suppression of HIV

This function defines the root equation for the triphasic model, i.e. V(t) - suppression_threshold = 0.

1	triphasic_root(timevec, params, suppression_threshold)

`timevec`	numeric vector of the times, t, at which V(t) should be calculated
`params`	named vector of all parameters needed to compute the triphasic model, V(t)
`suppression_threshold`	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.