apply_raw() for a simpler version of apply_ufunc()

apply_raw() would work like apply_ufunc(), but without the hard to understand broadcasting behavior and core dimensions.

The rule for apply_raw() would be that it directly unwraps its arguments and passes them on to the wrapped function, without any broadcasting. We would also include a dim argument that is automatically converted into the appropriate axis argument when calling the wrapped function.

Output dimensions would be determined from a simple rule of some sort:

• Default output dimensions would either be copied from the first argument, or would take on the ordered union on all input dimensions.
• Custom dimensions could either be set by adding a drop_dims argument (like dask.array.map_blocks), or require an explicit override output_dims.

This also could be suitable for defining as a method instead of a separate function. See https://github.com/pydata/xarray/issues/1251 and https://github.com/pydata/xarray/issues/1130 for related issues.

Start by reading the existing apply_ufunc() entry point and related issues #1251 and #1130. The work is done when the apply_raw() API, its dim-to-axis behavior, and a single output-dimension rule are agreed and implemented; the issue currently leaves those design choices open.