Refactor the code

[k-ye]

• [ ] GPU side implementation is messy, so is the code in unittests.
• [x] A lot of the code should be shared between CPU/GPU. Kernel functions, random floating point number generations (Done)
• [ ] Have a consistent name for the spatial hash
• [ ] Encapsulate GLShader and GLProgram into a separate class because soon we will need multiple GLPrograms for fluid rendering.