ImageMagick and PNM image support

Created by: laurentes

We are constantly facing problems across ImageMagick versions shipped with various Ubuntu distributions. In particular, we were not able to find a good way to deal with the portable anymap format (PNM) image formats (pbm, pgm and ppm), because ImageMagick behaviour is version dependent.

Furthermore, we are planning to rely on the (hopefully) more stable library libnetpbm to deal with these image formats.

As we have recently noticed behavior changes with the jpeg codec, we are also considering to get rid of the Imagemagick dependency for the IO codecs, in favor of more specific image libraries (such as libpng and libjpeg).