observables: make ids of ID based observable readonly.

[KaiSzuttor]

No description provided.

[RudolfWeeber]

If we decide we want to prevent users from changing what an observables observe over time, I think it needs to be done everywhere by making all the parameters read-only and only settable at construction, just like with correlators.
This PR changes the behavior for PidObservables, but looking at the code in src/script_interface/observables/ProfileObservable.hpp, one can also change histogram dimensions after the fact.
E.g., one an write to max_z of a density profile.

[KaiSzuttor]

yes, all of the observables should be immutable