THe growth force should depend on the cell size and the size of the
neighbors. However, only in the presence of neighbors, the growth should
be slowed down. Now, I could sole this, but have no longer included the
cell size, so that the repulsion does not correctly slow down the
growth.

The noise randomly increased the radius of non existing cells, thus,
filling the array.

Now, the cells repell each other (factor is not perfectly fine) and
equilibrate to a regular grid within short amount of time.

TODO:
- Allow isotropic cell growth (as a feature)
- Include apoptosis/mitosis

- Cells are initialized on a grid
- Cells grow with a constant force
- Growth is slowed down via a repulsive force between touching cells.
- Effective repulsion force is averaged over nearest neighbor sites in
grid.