Add parameter unit information

[JulesFleuren]

Motivation

Currently, (as far as I could find) there is no way to get the unit of a parameter. Although they can be found in the GRIB2 documentation, it would be convenient to also be able to get them with a built-in method.

Proposed Solution

The unit is obtainable in string form, containing the SI symbol of the unit. Two proposed solutions:

1. Make a method similar to the method for obtaining parameter names. For example, something like
   let unit = CodeTableUnits4_2::new(discipline, category).lookup(usize::from(parameter));

2. Include the unit in the parameter name. For example, something like:
   "Temperature [K]" instead of "Temperature"

Additional Context

I used this: https://www.nco.ncep.noaa.gov/pmb/docs/grib2/grib2_doc/grib2_table4-2.shtml as a reference for the units.

[noritada]

Thank you for making a feature request.

Adding unit information for each product was on my mental to-do list, but it wasn't a high priority.
Now that I understand there's a request for it, I'd like to raise its priority.