added ex04 solution

exercises/ex04_solution/exchange_energies_L6.png

exercises/ex04_solution/exchange_energies_L7.png

exercises/ex04_solution/heised.ipynb

exercises/ex04_solution/heised_cpp/.gitignore

exercises/ex04_solution/heised_cpp/Makefile

exercises/ex04_solution/heised_cpp/energy_gap.png

exercises/ex04_solution/heised_cpp/groundstate_energy.png

exercises/ex04_solution/heised_cpp/heised.cpp

exercises/ex04_solution/heised_cpp/openchain.dat

exercises/ex04_solution/heised_cpp/periodicchain.dat

exercises/ex04_solution/heised_cpp/plot.py