Nucleons do not return to center after going from unstable --> stable

[phet-steele]

Labeling as "dev:phet-io" since I do not see this bug in any production version.

1. From start, put two protons in the atom (any screen).
2. Check Stable/Unstable.
3. Take a proton out. The remaining proton will not be centered. Can happen with neutrons.

Easily seen with the center mark:

The phet-io reported values of that proton's position:

Seen on macOS 10.12.3 Chrome. For phetsims/tasks#792.

[phet-steele]

@jbphet, interestingly, this is not fixed after a reset all, or taking everything out of the atom.

[phet-steele]

Fixed, thanks.