Skip image generation for sentences if it already exist

[aoldoni]

• Although other images will change, sentence images will never change for a given word - e.g.: the sentences will always be the same.
• Add parameter to force uncached processing

[aoldoni]

Fixed in https://github.com/aoldoni/comp9596/commit/0f23de30f552f51ac3530e4485c6f32b744805ad