calc.exp.time: calculate the exponential time of the curve. This is done by...
In picousse/gRowthAnalysis:

calculate the exponential time of the curve. This is done by fitting a spline, and calculating when the first derivative goes below 0.

1 2	calc.exp.time(df = NULL, input_time = "time", input_value = "value", treshold = 0.3, lambda = 1e-05)

`df`	(dataframe). Dataframe containing the data
`input_time`	(string). Column name that contains the time value (in hours!). (Default = "time")
`input_value`	(string). Column name that contains the measured value. (Default = "value")
`treshold`	(double). End of exponential time is only calculated if this treshold has been surpassed. This mainly filters out the noise often seen in the beginnen of the curce.
`lamda`	(double), Smoothing paraneter. this needed to filter out outliers.