Sprint 3 Bug Fixing

Bug fixing log:

• Rewrote the unit tests for turrets as they were not working at all. This required introduction of mockito.
• Cleaned up unused imports accross the entire project.
• Fixed durasteel import typo on level 3.
• Fixed powerupfactorytest tests not working. They did not use mockito properly.
• Fixed treetops layer of map not being properly loaded or positioned on levels 2-3.
• Added irremovable flag to placeableEntities, preventing black boxes upon extractor removal. Includes a new unit test.
• Fixed new entities directory not properly being referenced.
• Fixed extractors mining at inconsistent rates.
• Balanced the rate of extractor production across the board.
• Resolved extractors being too large when placed by working with buildables team to introduce multi-size structures.
• Massive migration of all images to subdirectories of the images directory. Added tests to enforce no images remaining in images root directory. Resolved image import issues accross the entire project to use proper directory paths.
• Resolved level loading issue using java File instead of libgdx files, causing issues with gradle launcher.