Tukey's Tricube weight function
From the EGRET package http://usgs-r.github.io/EGRET/ Robert Hirsch and Laura De Cicco
numeric vector of distances from the point of estimation to the given sample value
numeric value, the half-window width, measured in the same units as d
Computes the tricube weight function on a vector of distance values (d), based on a half-window width of h, and returns a vector of weights that range from zero to 1.
w numeric vector of weights, all 0<=w<=1
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