Proposed changes

This PR performs hypersurface calculations for Klein-Gordon Cce.

The first commit does a worldtube transformation to convert the Cauchy variable $\partial_{t^\prime}\psi$ to the characteristic variable $\Pi$ (Cce::Tags::KleinGordonPi).

The third commit adds actions to compute the right-hand side of the hypersurface equation for $\Pi$. The equation reads

$(1 - y) \partial_y \Pi + \Pi = A_\Pi + (1 - y) B_\Pi$.

The second commit prepares necessary inputs to compute $A_\Pi$ and $B_\Pi$.

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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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