Clarify box origin-centered default in egs_box example

[dworogers]

In the egs++ manuals description of egs_box, it is not explicitly stated that the box is centred about the origin. That would be helpful. The example could also make clear with:

box size = 1 2 3    # ie from -0.5 to 0.5 cm x, -1 to 1 y and -1.5 to 1.5 z

[ftessier]

Thanks, this is spelled out in the egs_box documentation, but indeed adding a comment in the example would provide a useful reminder.