in config/formats/gadget.py : we avoid the problem generating the message :

this case is not taken into account, sorry !

Now, massnzero is fully compatible with npart and massarr.

git-svn-id: https://svn.epfl.ch/svn/pnbody@65 f1771ac9-52fb-4361-9f77-cbfb7563c7d4