create a functions to search for all continent population history data

[sahilsahil934]

1. Function to get all continent current year population : should return {"continent name": population, ...}
2. Function to get particular continent historic population and if extra year argument is passed show population for that year: should return {"continent_name": {"year": population, ....}}
3. Function for particular year all continent population: should return {"year": {"continent":population, ...}}