Compute ADM momenta as volume integrals

[iago-mendes]

Proposed changes

This PR improves the accuracy of P_ADM and J_ADM by computing them as a sum of infinite volume integrals and finite surface integrals, which were derived from the standard ADM equations using Gauss' Law. We have attempted to use this same trick for other asymptotic quantities (ADM mass/energy and center of mass), but the volume version didn't improve accuracy. This is evident from the convergence plot below, in which the CoM volume results have been omitted because of blow-ups.

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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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