Non-Accelerated Particle Simulation

[CaffeineViking]

For easier project building and better support/portability, we should also implement a software back-end for the particle simulation. Since the OpenCL kernel isn't especially complicated, it shouldn't be very hard to implement it (but might be hard to make they work alongside each other). I'll try to implement this in a weekend, by providing a build flag telling what back-end we should use (i.e. DISABLE_OPENCL) and also an extra flag telling (if OpenCL is enabled) which hardware accelerator should be used (i.e. CLDEVICE).