Default Base.keys(ds) to varnames(ds)

[lupemba]

• Add Base.keys(ds) = varnames(ds) keys have a default method.
• Define varnames instead of Base.keys for Datasets to align better with the interface description.
• Add small tests
  See issue for more context. Should Base.keys(ds) default to varnames(ds) #25

[lupemba]

There is errors in the integration tests.

• The TIFFDatasets.jl fail looks like an issue with GDAL and not something related to the PR.
• The GRIBDataset.jl fail might be related to the PR. I have tried to run the GRIBDataset.jl test locally from main using the PR as a dev dependency. The two test with errors in CI passes on my machine. Instead some other tests fails because I run windows.

[Alexander-Barth]

Thanks a lot, Simon!
Yes, the failures are indeed independent.

See also:
JuliaGeo/GRIBDatasets.jl#35
JuliaGeo/GDAL.jl#179