some reorganization: create ppStackUpdateHeader, ppStackStats, ppStackJPEG functions to clean up ppStackLoop; plug some leaks; cleanup include sections; create ppStackCleanup for end of processing (renamed old ppStackCleanup to ppStackCleanupFiles); move jpeg creation from ppStackCleanupFiles to ppStackJPEG; same for stats; adjust kernel auto-scaling to take place after we have measured the source sizes; update headers with stack_id, skycell_id, tess_id (optional)