Support for the `_Unsigned` attribute?

[asinghvi17]

Describe the bug

Values read by NCDatasets and ZarrDatasets were inconsistent with xarray for this file, because it was interpreted as Int16 data but the attributes had an _Unsigned = true key, so the element type should have been UInt16 instead.

using AWSS3, FilePathsBase
download(p"s3://noaa-goes16/ABI-L2-SSTF/2020/210/00/OR_ABI-L2-SSTF-M6_G16_s20202100000205_e20202100059513_c20202100105456.nc", "noaa.nc")
using Rasters, NCDatasets
Raster("noaa.nc")
# or whichever other way you care to load it

You can see the debugging history in rafaqz/Rasters.jl#735

To Reproduce

Please provide a minimal julia code example which reproduces the behavior (bug, performance regression, ...).

Expected behavior

I would expect that the array is somehow reinterpreted to the appropriate unsigned type.

Environment

• operating system:Mac M1
• Julia version: 1.10
• CommonDataModel version: latest

[Alexander-Barth]

Here is some previous discussion:

JuliaGeo/NCDatasets.jl#133

In short, this is a lot of work for some feature that never made it to the CF conventions and has been long
been superseded by native unsigned types in NetCDF4.

But if somebody would like to make a PR, I am happy to review it.
I guess that this can be used similar to other special attributes like scale_factor, units which affect also the element type of an array:

https://github.com/JuliaGeo/CommonDataModel.jl/blob/main/src/cfvariable.jl#L167

And add another function in the "transformation pipeline":

https://github.com/JuliaGeo/CommonDataModel.jl/blob/main/src/cfvariable.jl#L328

(but maybe I will find to do it myself should there be a shortage of volunteers... :-) )