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.  Compare from last point written to gcode.

git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@926 bbaffbd6-741e-11dd-a85d-61de82d9cad9