Add `.combine_embeddings()` method do DocumentArray

[JohannesMessner]

Right now it is painful to combine embeddings from different nesting levels and set them at the top level, especially until root_id is solved.

So this should do the following: It takes an access path (or list of access paths?) (e.g. @.[image, main_text]), and combines the embeddings in those docs using one of 'sum', 'mean', 'concat', or a provided model.