Add DataArray.insert_dim

[shoyer]

Signature: something like array = array.insert_dim(name, coord, axis=-1)

If index has size > 1, tile the array values along the new dimension, possibly using Variable.expand_dims to avoid copies.

[shoyer]

FYI you can currently work around this with:
xray.concat([array], dim=pd.Index([123], name='y'))

or for a dataset:
xray.concat([ds], dim=pd.Index([123], name='y'), concat_over=ds.data_vars)

[shoyer]

Related API: something like array.broadcast_to(other.coords) for broadcasting onto the coordinate system / indexes from another DataArray.

[shoyer]

expand_dims was added by #1347.