Add Skew control system to BBH

[knelli2]

Proposed changes

Part of #4603.

Depends on (but doesn't include) #6482 .

Upgrade instructions

Any input file for the BBH executable must now also have a Skew: block under the ControlSystems: group with all the same options as the other control systems.

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

[knelli2]

Still dependent, but now on #6493 (not included)