Added UnitTests for Lennard-Jones potential, improved ParticleContainer.

iterate_all_pairs_half was implemented and force calculators
modified accordingly.
ParticleContainer now includes a constructor and size method.
This way no friend is necessary anymore.

ParticleInput_FileReader and ParticleOutput_VTK were modified so
they don't need to be a friend of ParticleContainer.
Merge because of differences in files.mk (uncritical)

Conflicts:
	files.mk