Review of stereo item generation for P800-1 and P800-2

1. Added file designators in with naming convention of input files for P800-1 and P800-2.
2. Updated source file names, including correction for cat2_2 (both P800-1 and P800-2).
3. Fix for audiofile.write by addition of .wav file ending.
4. Outputting to category specific folders (e.g. proc_input/cat1 as expected by processing scripts).
5. Added multiprocessing, by default turned on.
6. Updated impulse responses for P800-2 cat2 to use the available ones.