PFM support from Gregor Mueckl

PFM is the 'float' flavor of PNM files.

I didn't write this, Gregor did.  But he forwarded the materials to me so I could do the pull request.

It LGTM, builds for me on both OSX and Linux, and I can load the test images he sent.

This is only for reading. It's probably straightforward to add write support, but Gregor has not done it yet. I'm not sure how often people will need to write float PNM, we can make it a priority if anybody needs it. Certainly, merging the read code only is still better than no PFM support at all.

I will let it sit for a couple days to give others a chance to comment on it, and if no objections are raised, I will merge it.