Adding geographical clarifications to REP-0103.

This pull request adds some clarification to REP-0103 about how systems aware of their global location and orientation should represent that information in standard cartesian ROS messages.

As the manufacturer of many outdoor, GNSS-equipped ROS systems, @clearpathrobotics is strongly interested in having clarity around these issues, as it helps us deliver value and consistency to our users, helps our users understand how their systems are configured, and helps everyone achieve harmonious integration between different IMUs, GNSSes, and INSes, as well as higher level localization and planning systems.

I'd prefer to save this follow-on discussion for later, but the larger context here is to work toward an amendment or follow-on to REP-0105 which would clarify what the meanings of the `odom` and `map` frames are in a globally-aware system, and whether a new frame is required which represents an absolute reference.
Introduce ENU acronym, which is used below.