Add compute tag for the second time derivative of the spacetime metric.

[ncorsobh]

Proposed changes

Adds a compute tag for the second time derivative of the spacetime metric in namespace gh. Computes this quantity in EvolveGhValenciaDivClean.

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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.

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