Added aggregate pattern for location service lab8

[atayyabi]

Two main changes implemented here are:

1. Move the implementation of get_all() and get(...) to the parent class and add a new abstract method to the parent class called get_location(). then we moved the get_location() method inside the child class in each module containing the child classes
2. changed the data.init.py module to load the DATA_SOURCES through a new method called getDataSources in services.location.init.py (so that each implementation details of the child sub classes are not leaked outside the module)