Control BBH masses and spins in initial data

[iago-mendes]

Proposed changes

I've implemented a Newton-Raphson iterative scheme with Broyden's method that controls the BBH masses and spins during the Initial Data. This follows a similar approach to what is described in section 2.3 of arXiv:1506.01689, except that here we control the Kerr masses and spins instead of the excision radii and horizon frequencies.

Upgrade instructions

If calling generate_id or id_parameters from InitialData.py, you now need to specify the masses mass_a and mass_b separately instead of the previous mass_ratio. Note that generate_id_command still takes in mass_ratio as an argument.

Code review checklist

• [x] 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.
• [x] 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

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