estimate
Estimate the sample size based on a normal approximation: the smallest whole n with z(1-alpha/2)*s/sqrt(n) <= desiredHW, and never less than 2, since a half-width needs at least two observations.
Return
the estimated sample size
Parameters
desired
the desired half-width (must be bigger than 0)
std
the standard deviation (must be bigger than or equal to 0)
level
the confidence level (must be between 0 and 1)