* Optimizations to FIF.cc. Unnecessary file header reading is now avoided as top
  level IIPImage class cache fully utilized. Cleanup of constructor code of IIPImage,
  TPTImage and KakaduImage code. Significant speed up to requests of cached images.
* Modification to if_modified_since code in FIF.cc, to avoid repeatedly resetting the
  timezone environment variable to UTC when checking timestamps, which is a relatively
  slow process. This is now set once globally in Main.cc and set back on main exit.