function of the release/debug drop down menu when using visual studio's IDE.

* Wether asserts are enabled or not, is now only suggested by dlib, and not longer imposed. The user can override using SET(CACHE FORCE) or command line -DDLIB_ENABLE_ASSERT=ON/OFF anytime.

* Fixed missing line in cmake for asserts.

now use target_compile_options().  This should avoid weird configuration
errors that happen in rare instances on some platforms.

messed it up and effectively disabled cuda in visual studio.  This fixes that.

was because of a bug introduced yesterday in the CMake scripts which would
accidentally pull in two versions of the OpenMP libraries, the GNU one and the
Intel one, but you can't mix them together at the same time.

also pushed all the cuda #includes into a .cpp file.

* temporary workaround for python hanging on dlib::shared_ptr_thread_safe<global_clock> when extension-module using dlib is unloaded on windows platform

* added some more doc and tests for the new delete_global_clock function. Minor adjustment to singleton pattern to allow easy access to singleton shared ptr

* alternate _WIN32 specific solution, with no extra functions nor user-actions needed

* removed floating/obsolete delete_global_clock, tab to space

* leaving linux code unchanged in static desctructor

* cleanup not modified files

* align formatting

rather than the entire tensor.

so they visit layers in the correct order.

Changed from ctrl to alt because otherwise it's easy to accidentally hit ctrl up and jump when you don't meant to while pruning an image list.

Made it more obvious that this file is a cmake tutorial.  Also added a few more comments about how to use cmake.

much faster when the off-diagonal vectors are all simple multiples of the
ones_matrix().

funny looking in some cases.