Fix output of quatu and quat in part print
VS_RELATIVE: Do not clear forces on virtual sites after transferring them to center of mass particle.
Forces are still cleared in init_forces() or the thermostat.
Implement pressure and stress tensor contribution for virtaul_sitse_relative (rigid bodies)
Fix compilation without vs_relatie
Merge remote-tracking branch 'rweeber/master'
Merge branch 'master' into cpp

Conflicts:
	src/pressure.cpp