Enable nested mapping as needed:

• only needed for embedded *-to-many with more than one field
• for these, the user should be able to opt out (nested would be the safe default mapping in this case, but they could want to opt out when only ever querying on single fields of the embeddable)

It might be a logic we could also use for the Lucene backend and if we came up with such a complicated scheme to determine if a field is going to be nested or not, we will probably need to integrate these information in the metada.