[ENH] PNMReadWriter ( [cd][er][sm] [approved] )

[ducasse at iam.unibe.ch]

The code looks good but I could not find a ppm with the right depth to
test it. 













< I'm a bug-fixing machine! >

This post brought to you by the BugFixArchiveViewer, a handy tool that
makes it easy to comment on proposed fixes and enhancements for Squeak. 
For more information, check out the Web page for the BugFixArchiveViewer
project: http://minnow.cc.gatech.edu/squeak/3214 

< I'm a bug-fixing machine! >