Refactor T_relation, T_clean_relation, and T_noisy_relation

[ThomasColthurst]

Right now our code has a large number of std::visit calls just to get the domains or is_observed bit of a T_clean_relation or T_noisy_relation.

These could be eliminated if we switched T_relation from being a std::variant into being a base class that would contain the domains and is_observed and have T_clean_relation and T_noisy_relation inherit from it.