Commit to merge with remote.

Ongoing work on output of stresses.
Merge remote-tracking branch 'origin/master'

Conflicts:
	LF_DEM/Simulation.cpp
Modified output of stresses:

1 - Fixed a bug of particle size dependance for monodisperse systems.
2 - Now, viscosity and stress contribution of contact forces are split
    between xF term and G*U_contact term
3 - Individual contributions are output for every particle in par_* file