Accessor Functions for root_rntuple_write_container Public Members #867

Move the public fields at https://github.com/Framework-R-D/phlex/blob/main/form/root_storage/root_rntuple_write_container.hpp#L61 into private fields with accessor functions. The accessor functions will just provide a reference to unique_ptr<> for now because root_rfield_write_container needs to initialize these members. These accessor functions have the bonus that we can update them with some extra initialization work in the far future.