Add spins to BBH ID input file

[nilsvu]

Proposed changes

Set horizon rotation parameters to a good guess based on the specified dimensionless spins, using Eq. (8) of https://arxiv.org/abs/1506.01689. Later we'll add outer iterations to control the rotation parameters within the executable.

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• 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
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• The PR lists upgrade instructions and is labeled bugfix or
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