don't skip triangles pointing down (z of triangle's normal is negative) - this was supposed to be an optimization, but it breaks further model transformations
 add check for invalid triangles to "binary" parser
 use "is" for comparing with "None"
 better wording: use "invalid" instead of "irregular triangle" (the English language is different from German :))


git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@98 bbaffbd6-741e-11dd-a85d-61de82d9cad9