made a minimum step distance for the Gcode exporter to allow configs for diferent resolution machines.  Also, the Gcode without this was making duplicate entries which causes the machine to stop and start inducing vibrations.

git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@925 bbaffbd6-741e-11dd-a85d-61de82d9cad9