get_nonparametricTTS: Non-parametric TTS function
In ushr: Understanding Suppression of HIV

This function computes the non-parametric form of the time to suppression

1	get_nonparametricTTS(vl, suppression_threshold, time, npoints)

`vl`	numeric vector of viral load measurements.
`suppression_threshold`	numeric value for the 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.
`time`	numeric vector indicating the time when vl measurements were taken.
`npoints`	numeric value indicating the number of interpolation points to be considered.