Remove confusion in gap interface conversion method

[user202729]

See the documentation changes in the issue below for an explanation why this change is correct. Basically _gap_ must take in an interface object and return element of that object; while _libgap_ must take no argument and return libgap element.

Also make IncidenceStructure inherit from SageObject — as noted in the documentation of SageObject, all objects visible to the user ought to inherit from it.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

#39905

Not strictly necessary, but useful to see why this change is correct.

[github-actions]

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