Added the boundary function.
Boundary calls added, but they break stuff.
Added velocity step stuff.
Added option to dispay the velocities in the grid.
Grid now hides the border cells.
Grid shows velocity cells again (easier to debug).
Fixed a bug in the diffuse code, and changed border.
Borders that are not negated are now zeroed instead.
Fixed some bugs.
TimeStep is now correct for the time interval.
Zeroed out prev values in grid each frame.
Advect is not working at all (does nothing...)
Changed back edge handling to normal.
Other bug fixes made this work now.
Added clearPrev function to Grid.
Using it to clean up each frame.
Somehow velocity works now, but too fast.
Changed back to my code (still works???)
Tuned some parameters, still not perfect.
Velocity field fades out too quickly.
Changed timestep to make it break:
too small a time step causes it to expload.