Testing version.

Correcting bugs on stress computation from resistance matrices.
In particular, XM was not computed properly.
Now, stress coming from 2-body forces are equivalent between resistance
matrix computation and direct pair evaluation s=xF.

Remains to be checked: one body part.
Merge remote-tracking branch 'origin/master'

Conflicts:
	LF_DEM/Interaction.cpp
	LF_DEM/Simulation.cpp
Merge remote-tracking branch 'origin/master'
Bug fixes on stress computation.

Now direct computation and resistance matrix based computation agree on
the stresses.

REMAINS TO BE CHECKED:
- sign (again, sorry) of Fc_normal to fix. The force behaves correctly
  now, but for clarity we should have Fc_normal>0.
- Normal stress differences with resistance matrix based stress
  computation. Does the traceless nature affect the normal stress difference?