Coordinate reference point seems to be in middle of area in laserfarm tutorial

[meiertgrootes]

the tiling (see #5) also results in cells with negative index values. In turn this seems to imply that the grid is extending towards negative values beyond the reference coordinate, both in x and y. This is possible, but usually we passed the lower left(?) bound as reference resulting in all positive indicees.

We should check what is going on and be explicit about this