Proposed changes

Add filtering to the GH executables. Note that filtering can be disabled in evolutions through input file options. This change is to support BBH evolutions.

Upgrade instructions

Input files for the generalized harmonic executables must provide options specifying what filtering to use (or to disable the filtering entirely).

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