Part of pywatts integration design questions.

pywatts uses xarray as central data container, while sktime does not have xarray as one of its supported input data types.

Adding it to the datatypes module would be very easy, but a few design questions need to be answered:

• should xarray become a core dependeny, or only a soft dependency?
• if only a soft dependency, how do we handle data types that are soft dependencies? We probably would have to insert some machinery in converters etc?
• should we introduce xarray for Series (1D, 2D), Panel (3D) and Hierarchical (4D plus), or only some of these?