Proposed changes

This is necessary to have the Gaussian centers move radially with the BNS. Rotation control already takes care of the angular movement, but since BNS doesn't use an expansion map, we need to track the object centers explicitly.

Upgrade instructions

The time dependent triple gaussian input file now needs:

        MovementMethod: ExpansionFactor

to preserve the same behavior.

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments