changed the gcode exporter code to allow a specific definition of the safety height (instead of previous "max(safetyheight, homeheight, startz)")
moved "startz" offset (based on the configured measurement unit) from SimpleGCodeExporter to the Gui code to allow more control
slight beautification of the gcode exporter code (added spaces between operators and after commas - according to PEP 8)


git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@165 bbaffbd6-741e-11dd-a85d-61de82d9cad9