AthContainers: Some updates to DataVector.
Some updates to DataVector, to be more consistent with recent standards.
- Declare some methods noexcept, where possible. (Some methods that are noexcept in std::vector do in fact throw exceptions with DataVector.)
- Declare empty as [[nodiscard]] as well as noexcept
- Have emplace_back return the pointer being pushed. For DataVector, we do the same thing for push_back,