* Semantic-segmentation loss calculation: fix buffer usage on multi-GPU training

* Review fix: make the work buffer live longer

Automatically test for NEON instructions when building python extensions and enable them if available.

Forgot about --yes USE_NEON_INSTRUCTIONS when removing --yes a moment
ago.  It's on by default now so now --yes really is unneeded.  The dlib
policy on this kind of thing going forward is to always have a cmake
script that tests if a feature like this is available and to enable it
automatically when building python extensions.

Removed --yes option from setup.py since it has long been a noop and its presence just confuses users.