Refactored ComputeLayer (#17)

• Removed deprecated init in ImageTexture and added a button to example project to save images to Photos

• Refactored source code

• Changed TextureOutput from a class to a protocol and added TextureIO which adopts both the TextureInput and TextureOutput protocol.
• Refactored the ComputeLayer class.
• Renamed Buffer to UniformBufferable.
• Renamed UniformBufferProvider to UniformSettings.
• Renamed UniformBuffer to Uniform.