Make the use of some null functions more consistent

[VincentVanlaer]

No description provided.

[Debraheem]

Is this just a simplification so that users can call the public star_data functions for making a pgstar decorator, as opposed to using a null other_* function? Is this easy for a random user to do? We generally point users to use the other_* functions, so we just want it to be clear how one adds their own decorator. I'm not sure if it's good or bad to do this. We don't want to hide this from users, and I don't know anyone that looks inside star_data/public, but maybe we can advertise it somewhere?

[VincentVanlaer]

I am not sure I fully understand your question? The only reason for the null functions to exist would be if they are used for initializing hooks. For the pgplot hooks in star and binary these are just initialized to null.

If you mean that these serve as an example for how to implement the specific hook, the interface definitions are basically the same. So perhaps that needs to be documented more clearly then?

[Debraheem]

Yes, my apologies, I mean the latter. I was just worried by the hook not being visible in star/other, but i guess it's a non issue.

We just want the interface format to be advertised somewhere in the docs, or mentioned. You've basically already done that, but maybe add the null interface example in $MESA_DIR/binary/public/binary_pgbinary.f90 as well.

Otherwise these changes look fine to me.