Add support for dask and xarray

[David Hoese]

Feature description

As discussed in pytroll/satpy#1910, calculations will generally perform better if they are wrapped in a da.map_blocks call when dask arrays are involved. The Satpy PR implements basic wrappers for this. In the long run it would be best if pyorbital had this builtin. @mraspaud had the idea of using a decorator that would detect if it was being given dask arrays or numpy arrays and wrap the function call in map_blocks if using dask arrays. Additionally, I think this functionality could be expanded to include xarray DataArrays.