added partial solution to ex05

exercises/ex05/solutions/BarnesHutTree.cpp

exercises/ex05/solutions/BarnesHutTree.hpp

exercises/ex05/solutions/Makefile

exercises/ex05/solutions/animated_configurations.py

exercises/ex05/solutions/benchmark.cpp

exercises/ex05/solutions/main.cpp

exercises/ex05/solutions/solar_system.cpp

exercises/ex05/solutions/solar_system.dat

exercises/ex05/solutions/test.cpp