added a "unit" parameter to the importers -> the svg/dxf importers can re-scale the model now, if the user configured "mm" instead of "inch" (pstoedit seems to output scaled to inch)


git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@608 bbaffbd6-741e-11dd-a85d-61de82d9cad9