More UPF cleanup

More upf cleanup: on-the-fly fix for UPF v.2 files containing undesired & characters moved one level up (it was inside read_upf, now it has to be called explicitly after read_upf fails). The rationale is to disentangle PP reading from parallelism (now parallelism is hidden deep inside PP reading), eventually moving all utilities for reading and converting PP's into a single, "almost stand-alone" library.