nucleonShellPositions arrays are incorrectly filled

[Luisav1]

The protonShellPositions and neutronShellPositions arrays in ParticleNucleus are incorrectly filled in cases where the nucleons are added rapidly after one another. I believe this might be because the array positions didn't get filled with incoming nucleons yet.

https://user-images.githubusercontent.com/52978048/211079555-1b376e6f-6666-4615-a7d9-c3ed4d0e9f96.mp4

In trying to track down this issue @marlitas and I found a few things:

• When a proton is added to protonShellPositions, it is also added to neutronShellPositions in the same xPosition and yPosition
  • In turn, this caused an incorrect oldNeutronCount of 1, placing the subsequent neutron as a continuation of the proton positions i.e. in position 2
  • Note: couldn't reproduce this after adding particleType to ParticleShellPosition (https://github.com/phetsims/build-a-nucleus/commit/b5a76000189d882789f484c67783cb9d186920ee)
• Nucleons sent to an older ParticleShellPosition whose particle property should be filled because the particle property is at some point getting overwritten to null, or maybe it was never filled to begin with?
• reconfigureNucleus() is getting called twice, which may be causing bugs, but this is fine on the DecayScreen where it is also called twice

[zepumph]

@Luisav1 said this is probably fixed, but I'll assign to @Luisav1 to confirm and then close. Thanks!

[Luisav1]

Can confirm this is fixed now. Closing.