ENH: For smaller array, added macro NPY_BEGIN_THREADS_THRESHOLDED in ndarraytypes.h
Avoiding NPY_BEGIN_THREADS for small arrays, can speed-up trivial_three_operand_loop by 5%.
As releases of GIL, then quickly restoring just after small operation doesn't benefit.
Merge pull request #3521 from arinkverma/gsoc_performance

ENH: Avoiding NPY_BEGIN_THREADS for small arrays can speed-up trivial_three_operand_loop by 5%