Compute scalar sources for Klein-Gordon Cce hypersurface equations

[Sizheng-Ma]

Proposed changes

In scalar-tensor theory, Cce hypersurface equations get additional source terms contributed by the stress-energy tensor of the scalar field. This PR adds and computes the corresponding tags.

The first commit adds a prefix tag Tags::KleinGordonSource for the source terms of the evolved variables (Tags::BondiBeta, Tags::BondiQ, Tags::BondiU,Tags::BondiW and Tags::BondiH). The second commit initializes the data storage in the databox for Tags::KleinGordonSource, as well as other tags that are required for later calculation. To compute the source terms, two quantities Tags::Dy<Tags::KleinGordonPsi> and Spectral::Swsh::Tags::Derivative<Tags::KleinGordonPsi, Spectral::Swsh::Tags::Eth> are needed. The third commit adds an action to compute their values. Finally, the last commit adds mutators Cce::ComputeKleinGordonSource to calculate the source terms.

Upgrade instructions

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

[wthrowe]

I don't know the internals of the CCE code well, but looks fine.