Adopted new changes on contacts updates in the Brownian time evolution.
Bug fix + small cleaning.

Bug fix:
	term xF for the contact stress was computed with a sign mistake.

Small cleaning:
	main change: I defined a Fc_normal_norm, and the former
	Fc_normal is now a vec3d containing the normal force acting on
	particle 0 from particle 1. This matches the def of Fc_tan,
	which is a vector.
Several modifications

1. Fc_normal_norm is now scaled by 1/shear_rate
2. Simulation is entirely shear rate independant for hydro part
3. Bug (Brownian stress) fix + warning fixes

I noticed that this version do not compute static friction any more. Is this
meant?