Compute spatial correlograms of spatial data or residuals.
    Syntax:
       correlogram(krig, nint, plotit=T, ...)
Input:
    krig trend-surface or kriging object with columns `x', `y', and `z'
    nint number of bins used
    plotit logical for plotting
    ... parameters for the plot

Output
    `x' and `y' coordinates of the correlogram, and `cnt', the
    number of pairs averaged per bin.

Side effect:
    Plots the correlogram if`plotit = T'.

Details:
    Divides range of data into `nint' bins, and
    computes the covariance for pairs with separation in each
    bin, then divides by the variance. Returns results for bins
    with 6 or more pairs.

See also:
    `variogram'

Example:
    topo.kr <- surf.ls(0,topo)
    correlogram(topo.kr, 25)
    d <- seq(0, 7, 0.1)
    lines(d,expcov(d, 0.7))