NumCosmoMathDataregister_shared

Gives data the chance to register, via ncm_serialize_set(), any of its own internal objects it wants shared (as an anchor, not deep-copied) by every later ncm_serialize_dup_obj()/ncm_serialize_to_variant() call made through the same ser — typically a large, read-only, per-instance payload that would otherwise be duplicated once per parallel worker. Subclasses only need to override this when they hold such a payload; the default implementation does nothing.