adding export pgm3D with cartesian image

We can have such 3D images with the display of DGtalTools Lib
![capture decran 2013-05-24 a 10 30 37](https://f.cloud.github.com/assets/772865/559386/41f3fc6a-c44c-11e2-87d6-3552d95a6c8f.png)