issue_comments: 328358162

I guess the key issue here is that some computations (E.g. finite differences) cannot be boiled down to passing one numpy function to atop. Instead, these calculations consist of three steps: 1) some dask calls, 2) a call to atop, map_blocks, etc, and 3) some more dask calls. In spencer's example, step 1 would be a the call to da.ghost.ghost and step 3 would be the call to da.ghost.trim_internal. While many dask functions have analogies in XArray, there are others that do not, such as da.ghost.ghost, so it won't always be possible to support this three step class of calculations by replacing the dask calls before and after atop with xarray calls.

@shoyer Would it unreasonably complicated to add some sort of pre_dask_atop and post_dask_atop arguments to apply_ufunc. Alternatively apply_ufunc could see if the func object has a pre_dask_atop method, and apply it if it does.