Change the GaugePlaneWave implementations for SO-CCZ4 in development.

[hen-w]

Proposed changes

Change the GaugePlaneWave implementations for SO-CCZ4 in development. The struct IntermediateVars is moved to public so one can easily construct the h function and its derivatives.

Upgrade instructions

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

[nilsdeppe]

LGTM, one small request. You can squash immediately.

[nilsdeppe]

Here the move is better for the unique ptr. For the array it doesn't matter. It's possible to elide a copy by taking the pointer by value and then moving.