Implement multi-threading for SIMD-accelerated dot product functions

Implement multi-threading support for SIMD-accelerated dot product
functions in dstSIMDDotMatrix.cpp, using the dstThread.h interface.

The dstThread.h (which implements dstTaskScheduler) has been debugged,
changed, extended and tested. It stil relies on repeated creation of
threads for multi-threading operations. Ideally, a limited number of
threads should be kept alive awaiting activation for a new calculation
to avoid the overhead of repeat thread creation.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>