Switch to DimensionalData

[felixcremer]

We would like to switch the internals of YAXArrays to DimensionalData

• Add keyword argument to define output array type to open_dataset
• update toaxisin Dataset.jl to construct Dim types
• How to handle CleanMe to finalize datacubes that are out of scope, but have been written to disk?
• make mapCube return a DimArray type

[felixcremer]

This is closed by #249 where we made the YAXArray a subtype of DimArray.