This can be a flexible tool for certain effects. In my case, it is
having a single element in the centre (achieved with ``e.fixed = 1``),
whilst everybody else tries to be a bit far away, but not too far away.
So, I have a moat field where there is centrifugal force if you are more
than R away from centre, and centripetal force if you are less than R.

As a special case (and the default), this is the traditional gravity
field.

This reverts commit cdbdba9.

This was rejected: #519

Update to newer version