Calculates the Geothmetic Meandian as defined by xkcd.com/2435

M = geothmetic_meandian(X)
M = geothmetic_meandian(X, dim)

M is given by computating 3 parameters: mean(X), geomean(abs(X)) and
median(X), and iterating the computation until the 3 parameters converge.
The parameters are considered converged if their range < 1e-10 * range(X).
The final M is the mean of the converged parameters.

input:
X: input array
dim: dimension on which calculations are applied (default = first non-singleton dimension)