Custom pnmio

[nnormand]

Thanks a lot for contributing to DGtalTools-contrib, before submitting your PR, please fill up the description and make sure that all checkboxes are checked.

PR Description

The dependency to libNetPBM was removed for PBM images input and PGM output. It is still possible to read/write PNG images using LibPNG.

Checklist

• [X] Doxygen documentation of the code completed (classes, methods, types, members...)
• [x] Check if it follows the tools structure described in CONTRIBUTING.md
• [X] New entry in the ChangeLog.md added.
• [x] Update the readme with potentially a screenshot of the tools if it applies.
• [x] No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).

[kerautret]

thanks @nnormand, could you please update from master and push ? (I have changed the url to get boost and I can't reload travis due to a bug I think)

[kerautret]

looks fine merging tanks!