allow dask.array in xarray_apply_along_time_dim

[cchwala]

There is a limitation of the current implementation of the xarray_apply_along_time_dim that is used to decorate all processing functions to allow seamlessly passing xarray.DataArray as arguments and then applying the functions along the time dimension.

The underlying xarray.apply_ufunc needs to explicitly allow dask.arrays which it currently does not. When I wrote the code, I has some problems figuring out how to do that. Hence, I left if like it is now.

This limitation should be removed, or it should at least be documented... :see