Boundary component for Klein-Gordon CCE

[Sizheng-Ma]

Proposed changes

This PR adds a boundary parallel component KleinGordonH5WorldtubeBoundary for Klein-Gordon CCE. The component has a single initialization action Actions::InitializeWorldtubeBoundary<KleinGordonH5WorldtubeBoundary<Metavariables>>, which initializes two tag lists Metavariables::cce_boundary_communication_tags and Metavariables::klein_gordon_boundary_communication_tags ( the metavariable will be added in the next PR). The third and fourth commits of this PR add the initialization action and the component respectively.

The initialization action is constructed from a base class InitializeWorldtubeBoundaryBase, which currently holds a single tag list. This is fine for the current CCE system since the code treats Metavariables::cce_boundary_communication_tags as one ::Tags::Variables. However, for the upcoming Klein-Gordon CCE system, we want to handle Metavariables::cce_boundary_communication_tags and Metavariables::klein_gordon_boundary_communication_tags separately, so we initialize them as two different chunks of ::Tags::Variables. In the first commit, a tag pack is added to InitializeWorldtubeBoundaryBase for this purpose.

The second commit is a minor change. The function create_fake_time_varying_klein_gordon_data is moved to a new file so that it can be used by other unit tests.

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

[knelli2]

With just this commit, this actually adds a Variables tag with an empty list since there are no extra tag lists yet (which is not something we want). I'd recommend something like

template <typename Initializer, typename ManagerTags,
          typename... BoundaryCommunicationTagsList> {
  // other stuff

  simple_tags = tmpl::list<::Tags::Variables<BoundaryCommunicationTagsList>...>;

  // inside apply function
    Initialization::mutate_assign<simple_tags>(make_not_null(&box),
                     Variables<BoundaryCommunicationTagsList>{
        Spectral::Swsh::number_of_swsh_collocation_points(l_max)}...);
};

Not that we'll likely need more than 2 lists, but this also avoids the annoying 1 and 2 suffixes.

[Sizheng-Ma]

Good idea!

[knelli2]

You need to add the hpp too.

[knelli2]

We don't have the initialization_tag type alias anymore, it's called simple_tags_from_options now. I know it's incorrect for the other ones as well. Could you please fix those dox in a separate commit?

[Sizheng-Ma]

Done!

[knelli2]

Looks good. You can squash

[Sizheng-Ma]

Done!

[knelli2]

Test timeouts unrelated.