getattributes(::DimArray): always a Dict with String keys

[bjarthur]

The axis interface says getattributes returns a Dict keyed by Strings. For a DimArray the DimensionalData extension returned metadata(x) as is, which is NoMetadata() for an array built without metadata, or a Metadata wrapper, or a Dict keyed by Symbols.

Downstream, YAXArrays stores that object as the YAXArray's properties, and savedataset then fails with MethodError: no method matching copy(::NoMetadata). Users work around it by constructing the DimArray with metadata = Dict{String,Any}() (see JuliaDataCubes/YAXArrays.jl#362 and its MWE).

This normalises the three cases to Dict{String,Any}: NoMetadata to an empty Dict, a Metadata wrapper to its Dict, and any AbstractDict to one with String keys. Anything else is passed through unchanged.

julia> using YAXArrayBase, DimensionalData
julia> getattributes(DimArray(rand(2, 3), (X(1:2), Y(1:3))))
# before: NoMetadata()          after: Dict{String, Any}()
julia> getattributes(DimArray(rand(2, 3), (X(1:2), Y(1:3)); metadata = Dict(:units => "K")))
# before: Dict(:units => "K")   after: Dict{String, Any}("units" => "K")

Tests added to the DimensionalData test item for all three cases.

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