Delete the method _parse_other in the Ad operator class

[keileg]

The method pp.ad.Operator._parse_other() is almost surely never used in practice; rather it is a remnant of an older approach to the parsing. We should verify that it is actually not used, and purge if possible. This should not be a major task.

This will be a preparatory step towards a full rework of the Ad operator parsing. Though the purge can be done together with a larger rewrite, it is preferable to do it before, so that the code is a bit cleaner when we move to the larger part.

[mikeljordan]

Quick Look at the Operator class: The _parse_other() method is only used through implicit calls in the operator overloading methods (e.g., add, mul, etc.). This is a legacy approach and should be reviewed for potential refactoring or removal if the operator overloads methods are not used.

[keileg]

If the original observation, that this method is not used, is wrong, I suggest we close the issue.

[mikeljordan]

On a final note, the method _parse_other() cannot be removed because of the implicit calls to it by the overloading methods within the operator class which are inherently used whenever there are operator arithmetics elsewhere in the codebase. Removing it would mean code duplications (dumping its implementations where it's called), which I think deviate from what we want to achieve.