Add a radial expansion to the BBH ID pipeline

[knelli2]

Proposed changes

Upgrade instructions

For the spectre bbh generate-id command, you must now also specify the radial expansion velocity with --radial-expansion-velocity/-a.

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