Other functions than `create` should also return `subfeed` keys

[Powersource]

create currently returns this (copied from readme)

• cb Function - callback function of signature (err, group) where group is an object containing:
  • id CloakedId - a cipherlink that's safe to use publicly to name the group, and is used in recps to trigger encrypting messages to that group, encoded as an ssb-uri
  • subfeed Keys - the keys of the subfeed you should publish group data to
  • secret Buffer - the symmetric key used by the group for encryption
  • root MessagedId - the MessageId of the group/init message of the group, encoded as an ssb-uri.

Functions like group claim that they return the same format

• cb Function - callback function of signature (err, group) where group is an object on the same format as the group object returned by #create

but they don't currently return subfeed. It's not stored in box2/keyring.

[Powersource]

This gets messier now that we have epochs