Fixed constructor bugs introduced earlier.
Fixed atom type in addAtom() and addGhostAtom() in atommanager.cpp.
Made twoParticleAction public in lennardjonespotential.h.
Added atom type in addAtom and addGhostAtom in system.cpp
Switched order of ghost atoms and normal atoms in mpi copy function.
Should not really affect anything.
Added working forces test for lennard jones.
Fixed tests for system (add/remove atoms, number of ghost atoms) with
the changed behavior of atomManager.
Nothing important.