Fix injection of added parameters

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGELOG.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

The issue was that a YAML definition of an indicator was not able to inject (set) the indexer parameter on indicators that inherited from IndexingIndicator (i.e. indicators that rely on a class-level implementation of the indexing, instead of compute-level one).

This reorganizes how these class-added arguments are handled. It also renames them "added" arguments instead of "injected" to clear the confusion. "injection" now only means an indicator argument that is set when creating a derived indicator (that what most YAML definitions do). "added" is an argument that is not in the compute function, but added by the class.

The example YAML is modified and a test is added using this modification.

Does this PR introduce a breaking change?

No.

The feature was not really documented and now it works instead of not.

Other information:

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[coveralls]

coverage: 89.377%. remained the same
when pulling d2be937 on fix-indexer-injection
into 5bf3e9b on main.