Proposed changes

The files src/PointwiseFunctions/AnalyticSolutions/GeneralRelativity/KerrHorizon.xpp are more general than the GeneralRelativitySolutions lib and therefore should be moved to GeneralRelativity lib. This will also remove the unintuitive dependency of DomainTimeDependence lib on GeneralRelativitySolutions lib.

Upgrade instructions

Change the include directory for KerrHorizon.xpp files to PointwiseFunctions/GeneralRelativity/KerrHorizon.xpp.

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