Add identification of a dimension by units

As of now, ClimaAnalysis identifies the dimension by a list of provided names. However, according to section 1.4 in the CF conventions, "Latitude, longitude, and time coordinates are identified solely by the value of their units attribute". The units to check would be `degrees_north`, `degrees_east`, and `seconds since blah`.

It would be good to use UDUnits for this. See issue #209 for this.

No file or test is named in the issue, so start by locating the current dimension-identification entry point and read issue #209 for the UDUnits context. Check how CF units such as degrees_north, degrees_east, and seconds since blah are handled, and consider the work complete when dimensions can be identified from those units as well as existing names.

Written by the indexing model from the issue text.