PPM encoding/decoding

[xd009642]

I found during my median filter implementation that there is some issue in ppm encoding/decoding. This may be due to 16 bit width PPMs or other extensions. Or mishandling of the format.

Issues seems to partially be down to the ppm being larger than the header suggested it should be. Which is now sorted. However, there are still some issues with this random ppm I've downloaded that I've not encountered before - but could be related to the fact it's 1 byte too large.