added serialization of electron shell positions to fix issues with phet-io state setting, also did some code cleanup, see https://github.com/phetsims/build-an-atom/issues/107
added serialization of electron shell positions to fix issues with phet-io state setting, also did some code cleanup, see https://github.com/phetsims/build-an-atom/issues/107

phetsims/tandem,phetsims/tandem,phetsims/tandem