fix: changed arraylist to interface list where possible

What are the key features of this change

Changed ArrayList to the more general List to be able to easily change between different types of lists. ArrayList are still used when creating objects. Additionally javadoc was added to display the refactor for army, armyfilehandler and List mentions in javadoc

closes: #22 (closed)

Size of change: small

Checklist

• Javadoc is added to methods and classes
• Tests are added to feature
• Build tool test passed