Move PPM/PFM I/O functions to the library

[walbourn]

The PPM/PFM functionality is currently part of the texconv tool. I don't believe it's an important format to support as part of the mainline library, but it could be moved to the Auxiliary folder instead of being side-by-side with texconv

[walbourn]

The auxiliary idea is problematic, so I think it would better to just make PPM/PFM a primary library module.