Add apply_gufunc

[tomwhite]

This is for #67

This is a cutdown version of the equivalent code in Dask. It passes a number of tests, but the main limitation is that it only supports a single output. I've got a number of ideas for removing this limitation (e.g. write one output at a time - in series), but I think this is a good start for supporting xarray.

@TomNicholas I'll leave this open for a few days in case you want to take a look!

[TomNicholas]

Awesome @tomwhite !

My only comment is that when xarray.apply_ufunc is called with dask then output_sizes is a required kwarg (at least when creating new dimensions on the output IIRC). I would need to look at the implementation in xarray again, but it might be the case that output_sizes would be required for xarray to make use of cubed.apply_gufunc too.

[tomwhite]

Thanks for taking a look @TomNicholas and pointing out that output_sizes is needed in xarray.

I'm going to merge this and create a follow-on issue for output_sizes (see #150), as it needs a bit more investigation.

[tomwhite]

Support for output_sizes is in #151