BBH pipeline: find horizons in initial data, write masses & spins to file

[nilsvu]

Proposed changes

Automatically runs the horizon finder over initial data and writes horizon quantities to the reductions file. This makes important quantities like horizon masses and spins available to later steps in the pipeline.

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

[nilsvu]

@knelli2 no I decoupled this from #6090. This is ready when you are :)