[Gmsh] Modification of .stl file

[Geordie McBain]

Besides D. L.'s suggestions (I can confirm Blender will certainly do
the job), another way to do Boolean operations on StL surfaces is with
GTS <gts.sf.net>.  The GTS format is equivalent to the StL for
triangulated surfaces and comes with stl2gts and gts2stl converters.
It also has a "set" program for the operations you want.  Some time
ago I wrote an example of just what you wanted: how to drill a
circular-section hole through a solid defined by a triangulated
surface at http://gdmcbain.freeshell.org/fluids/gerris under the
heading `The GTS set tool'.  Hope this helps.