Add tests to the core module

[ghacupha]

Expected Behavior

We need to increase test coverage in the core module, on which all other modules depend

Current Behavior

The current coverage for the core module is at 76%. Room for improvement.

Possible Solution

-Stick to Junit as amuch as possible. Mockito may not be necessary. We can generate class and XSDs to do round-trip tests where we are using JAXB api.
-The code has a lot of static methods, supposed to make use of the api easier. This consists the bulk of useful untested code. We could instead initiate such classes using double-check locking singletons, turn the static methods to instance methods.

• Use dependency injection pattern instead of configuring dependencies inside the classes. This would make the code more testable and maintainable.
• As much as possible reduce side effects