Add parallel variables/constraints to `report_numerical_issues`

[Robbybp]

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Right now, display_near_parallel_* methods are considered "advanced diagnostic techniques" and only suggested if report_numerical_issues finds no issues. However, I would argue that they should be considered "numerical diagnostics" as they run quickly, give concise output (for an appropriately tight tolerance), and don't depend on external solvers. I propose we add them to the "numerical warnings" section of the diagnostics toolbox. I am happy to open a PR if others agree.