Proposed changes

This PR implements the $J_{ADM}$ integral based on arXiv:1506.01689. We only compute the z-component because that's the quantity of interest to control hyperbolic encounters. Since the $J_{ADM}$ integrand is closely related to the $P_{ADM}$ integrand, I've put both pointwise functions together, renaming AdmLinearMomentum to AdmMomentum.

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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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