Consider -Wdeprecated-copy-with-user-provided-copy warnings #1498

As reported in #1485, -Wextra reports this kind of warning. As discuss there, rather than adding an explicit default copy constructor, it would probably better to just remove the custom assignment operator. If I'm correct, if we remove it, the compiler generates a default one that delegates to the base class, which already handles the assignment nicely (i.e. shares the underlying SEXP, which has the tzone attribute already).