(Closes #2622) Add convenient methods `SymbolTable.insert_argument` and `.append_argument`.

[JulienRemy]

Arguments currently have to be inserted in SymbolTable().argument_list using a workaround:

• 1. getting it, which checks the arguments and non-arguments symbols in the table,
• 2. adding the symbol to the table and the argument list. Trying to do it in the reversed order causes a validation error on either the table or the argument list.

This introduces SymbolTable().insert_argument(index, argument) and .append_argument(argument) to hopefully cover most use cases.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.86%. Comparing base (647447e) to head (9c966a9).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2626 +/- ## ======================================= Coverage 99.86% 99.86% ======================================= Files 356 356 Lines 48457 48483 +26 ======================================= + Hits 48392 48418 +26 Misses 65 65 ```

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[JulienRemy]

@arporter @sergisiso What do you think about these?

[JulienRemy]

@arporter I edited w.r.t. all the points above, except for unindenting as I get Flake8 complaints if I do. I also formatted the docstrings of the neighboring methods (no \, indentation) and added my name in the Modified or Authors sections, which I always forget to do.

[JulienRemy]

asserts are now unindented and I also changed the _validate_... error strings to use type(object).__name__.