Returns the age-interpolated median of the GCDG references (Weber
et al, 2019). The working range is 0-4 years. This function should
be called when the
key = "gsed" or
key = "gcdg".
Decimal age, numeric vector
A vector of length
length(t) with the median of the GCDG references.
Internal function. Called by
dscore:::count_mu_gcdg(0:2) #>  3.308477 49.244553 62.480171