Add units to ascii2nc output where possible.

[JohnHalleyGotway]

Describe the Enhancement

While working on #2426, I discovered that ascii2nc never writes the units string for point observations to the obs_unit NetCDF output variable. The pb2nc tool does write the unit strings. This issue is to enhance ascii2nc to parse and write the units of the observations to the output.

Note that this will vary considerably based on the input formats and more definition is needed.

ascii2nc should keep track of the units string for each observation type and print a warning message if that string changes during a single run.

List of input formats:

• NDBC files do define the units.
• Add details for other input file types here.

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Issues should represent approximately 1 to 3 days of work.

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