added solutions

...s/Pandas_GroupAssignments/AachenBleifuesse_solution.ipynb

...ndas_GroupAssignments/First-Year-Students_solutions.ipynb

Exercises/Pandas_GroupAssignments/GrainUsage_solution.ipynb

..._GroupAssignments/HistoricalWeatherData_1_solutions.ipynb

..._GroupAssignments/HistoricalWeatherData_2_solutions.ipynb