Support `allow_rechunk=True` in `apply_gufunc`

[tomwhite]

This is needed by interp in Xarray.

[dcherian]

FWIW I'd actually like to delete this kwarg and have Xarray issue a warning and explicitly rechunk instead of passing it down.

[tomwhite]

That makes sense as there's only one reference to allow_rechunk=True in Xarray (in interp).

[dcherian]

yes and we always want users to do it explicitly. The only reason it's there now is for backcompat after I did a major rewrite.