Sph: add experimental SPH fluid-soft body demo; fix SphHf demo.
(One way interaction - fluid reacts to soft body, without CCD).

Fix: SPH particles at the edges of the btHfFluid did not
contribute fluid in the SphHf demo.
Sph: fix multithreaded performance scaling.
(Simultaneous execution of btParallelFor::mainFunction() and
 btThreadSupportInterface::waitforResponse() on the same core
 leads to constant context switching.)

Timings for 'Demo_ManyParticles', 65536 particles, Phenom II x4 2.8Ghz;
btFluidSphSolverMultithreaded::updateGridAndCalculateSphForces():
	~151ms - 1 thread
	~97ms - 2 threads
	~77ms - 3 threads
	~65ms - 4 threads
Sph: fix AABB in softbody demo, draft 2 way fluid-soft interaction.
(2 way interaction is not enabled by default, toggle
 APPLY_IMPULSE_TO_SOFT_BODY in FluidSoftInteraction.h to enable).