Update Grid.js

[rbonghi]

Now the grid use option in SI.
Fixed size

[T045T]

I'm not sure this is an improvement.

For example: What if size isn't a clean multiple of cellSize? Seems like you'd end up with a Grid that's not actually the size you specified in the options, wouldn't you?

[rbonghi]

Yes, I forgot this problem.

I suggest to change the name size in num_cell in according with same name in rviz application.

[rbonghi]

I have renamed size in num_cells. Now it's more readable.