rh_corrected_nephelometer values are set to NA if they are negative or
if the RH at the same time is higher than 90%.
rh_corrected_nephelometer values are also corrected for the HEPA zeroings (start and end, if there were done):
if a stable period longer than 3 minutes can be identified for the HEPA period,
using the changepoint cpt.mean
function, there is a zero value.
There can be no zero values, only one (beginning or end) or two.
If there is only one zero value, it is substracted from all rh_corrected_nephelometer values.
If there are two, a linear interpolation is done between the two values and the resulting vector
is substracted from the rh_corrected_nephelometer values.