fixes the bug that causes sort grid to draw a -999, -999 crosses for the...

this bug caused Sortgrid (in the calibration) to draw crosses at -999, -999 for the missing points
the simple fix is: 
        x = [[i for i in x[0] if i > 0]]
        y = [[i for i in y[0] if i > 0]]

unfortunately the file used tabs instead of spaces, so this is why you see "a lot of changes" below. The only real change is at line 446:

https://github.com/alexlib/openptv-python/compare/fix_bug_sortgrid_crosses?expand=1#diff-a4c26d43ca4149894ca3fb325dda91a8R446