Add a new broadcasting mode that inserts size-1 dimensions.

[dcherian]

I've found it useful to broadcast to size-1 unlabeled dimensions. This allows you to avoid useless extra work in functions that accept unlabeled arrays, and would be very convenient when paired with apply_ufunc.

For example: https://github.com/xarray-contrib/flox/blob/ca576812e78b3978421eace6e9dde5a76729ebcc/flox/xarray.py#L45-L62

This should be paired with #2171

[TomNicholas]

Wouldn't it be cleaner to just stop supporting unlabeled arrays in apply_ufunc? See #8995

[TomNicholas]

Or are you saying #2171 shows there is a use case for broadcasting to size-1 unlabeled dimensions outside of apply_ufunc?

[dcherian]

The function being applied could benefit from receiving broadcasted arrays with size-1 dimensions instead of size-N. #2171 is relevant in that we'd need it for DataArrays broadcasted in this manner to be actually useful in Xarray ops.