Colocated multigrid

    +---+---+---+---+---+---+---+---+---+
 8  | x | x | x | x | x | x | x | x | x |
    +---+---+---+---+---+---+---+---+---+
 7  | 0 |   |   |   |   |   |   |   | x |
    +---+---+---+---+---+---+---+---+---+
 6  | 0 |   |   |   |   |   |   |   | x |
    +---+---+---+---+---+---+---+---+---+
 5  | 0 |   |   |   |   |   |   |   | x |
    +---+---+---+---+---+---+---+---+---+
 4  | 0 |   |   |   |   |   |   |   | x |
    +---+---+---+---+---+---+---+---+---+
 3  | 0 |   |   |   |   |   |   |   | x |
    +---+---+---+---+---+---+---+---+---+
 2  | 0 |   |   |   |   |   |   |   | x |
    +---+---+---+---+---+---+---+---+---+
 1  | 0 |   |   |   |   |   |   |   | x |
    +---+---+---+---+---+---+---+---+---+
 0  | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | x |
    +---+---+---+---+---+---+---+---+---+
      0   1   2   3   4   5   6   7   8

- Pressure is defined on all cells:
	- The 0 cells are stored but not used (we return p = 0 on these cells)
	- The x cells are not stored (we implicitly return p = 0 out of bounds)

- Velocity is only defined in the inner cells, we always return 0 on the borders

The Coarse grid is as follows:

  +-------+-------+-------+-------+-------+
  |       |       |       |       |       |
  |   x   |   x   |   x   |   x   |   x   |
  |       |       |       |       |       |
  +-------+-------+-------+-------+-------+
  |       |       |       |       |       |
  |   0   |       |       |       |   x   |
  |       |       |       |       |       |
  +-------+-------+-------+-------+-------+
  |       |       |       |       |       |
  |   0   |       |       |       |   x   |
  |       |       |       |       |       |
  +-------+-------+-------+-------+-------+
  |       |       |       |       |       |
  |   0   |       |       |       |   x   |
  |       |       |       |       |       |
  +-------+-------+-------+-------+-------+
  |       |       |       |       |       |
  |   0   |   0   |   0   |   0   |   x   |
  |       |       |       |       |       |
  +-------+-------+-------+-------+-------+

So, the size of the total grid is (2^N+1)*(2^N+1), but we store only (2^N)*(2^N), and explicitly
set the pressure to 0 on the lower and left borders.