adding PNMReader
 add ppm sample
 changellog
adding functor to PNMReader and split PNM into PGM/PPM
adding functor to RawReader
adding functor to VolReader
adding functor to MagickReader and longVolReader
adding functor to GenericReader
debug options magick
modif functor type hdf5
Merge branch 'master' of github.com:DGtal-team/DGtal into PPMReader
David reviews.
addition of header
Merge pull request #631 from kerautret/PPMReader

Ppm reader
Minor doc edit
merge from master