Clarify Data Variables and Variables Containing Coordinate Data

[jamesdoyle21]

CF 1.12
Issue #501: Clarify that data variables and variables containing coordinate data are highly recommended to have long_name or standard_name attributes, that cf_role is used only for discrete sampling geometries and UGRID mesh topologies, and that CF does not prohibit CF attributes from being used in ways that are not defined by CF but that in such cases their meaning is not defined by CF.

Ben comment,
"First part is at least partially implemented, will need to check code. Second part we could omit or just throw a warning."