* Optimize appending another Parts container
* Remove redundant/verbose comments
* Change r-value args to move-only types into l-value args for
readability
* BREAKING CHANGE: Remove `AtRef(int)` and `AddPart(Message*)` member functions
* Add various const overloads
* Add `Empty()` and `Clear()` member functions
* Add `noexcept` where applicable
Distribution policies forbid installation of environment scripts at the
install prefix. So, e.g. in the Fedora DDS package the DDS_env.sh is
installed to /usr/bin and available via $PATH.