No delta time parameters

[vidstige]

In the original paper a delta-time parameter is used, this allows duration to be expressed in seconds rather that frames, and also the viscosity and velocities and other vectors can have more sane units. In my humble opinion pixels/frame^2 is trickier to think about compared to pixels/s^2... 🤔 Furthermore, after studying the code it should be fairly easy to add a delta time parameter to both advect_diffuse() and project() functions. Possibly with default value of dt=1.