mark_sig_color - Set the color of an edge based on p-value

[sfcheung]

setcolor has been drafted but not yet ported to the package.

Do this in its own branch.

[sfcheung]

It's low priority because some journals still publish black and white graphs (though more and more journals accept graphs in colors).

[sfcheung]

Changed the tentative name to set_color, to be consistent with the current style in naming the functions.

[sfcheung]

This function should be able to set three colors, one for significant and positive paths, and one for significant and negative paths, and one for nonsignificant paths.