Add option to skip ghost force reduction

[capomav]

Fixes #4556

[RudolfWeeber]

Lgtm so far. Next step are

• skip setting up the collect_ghost_forces_comm in the constructor of RegularDecomposiotion (srccore/cell_systems/RegularDecomposition.cpp) if the flag is ture.
• Modify the testsuite/python/lj.py so that it runs one test without skipping the ghost force reduction and another with skipping it. That second test will fail.
• check RegularDecomposition::init_cell_neighbors() to make sure, that the neighborship of ghost cells is set up correctly
• Modify the short range loop to also run on pairs of ghost cells and their red_neighbors.