re-organize ppStackLoop to make the steps clearer: add separate functions for UpdateHeader, JPEGs, Stats; move cluttered includes into ppStack.h; push DIE section of ppStack.c into ppStackCleanup (like ppImageCleanup); rename old ppStackCleanup to ppStackCleanupFiles; consolidate file cleanup block in ppStackLoop (for unconvolved outputs) into ppStackCleanupFiles; add options for stack type