WorldtubeBufferUpdater and WorldtubeDataManager for CCE

[Sizheng-Ma]

Proposed changes

This PR aims to pave the path for dealing with the worldtube data of the Klein-Gordon CCE system. Two major changes are introduced to the current infrastructure

1. The current WorldtubeDataManager is hard-coded to handle Tags::characteristic_worldtube_boundary_tags<Tags::BoundaryValue>. This can be relaxed so that it can be used to deal with other worldtube data (say a scalar field).
2. There are repeated contents in WorldtubeBufferUpdater and WorldtubeDataManager, and will be more once they are extended to handle the Klein-Gordon system. I extract some common parts into functions to avoid more copy and paste.

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

[Sizheng-Ma]

I've tested the executable CharactersiticExtract with a BBH system. All the waveform outputs are not impacted by the changes.

[Sizheng-Ma]

Done!

[knelli2]

Looks good. Squash all the fixups including these last few changes.

[Sizheng-Ma]

Done. I noticed that the first argument of set_time_buffer_and_lmax was also a reference. I switched it to gsl::not_null in the last commit.

[knelli2]

Nice catch. You can squash the final fixup

[Sizheng-Ma]

Ok, squashed.