This creates a particle_type=True Temperature field using the definition Junhwan supplied in his message to the mailing list.
In addition, I've wired up an alias from this new field to the `('gas', 'temperature') field.
Lastly, I've removed a vestigial cython routine from the SPH frontend. This is all dead code at the moment.
Technically this is a new feature, but I personally don't see any issues with including this in 3.1.
All tests pass. Good job!
Looks like minor conflicts in a deleted file, but I am okay with you pushing this manually.