Radix Messaging to serveral receivers

[ARlayers]

To enable sending a message to multiple recipients, I would suggest the following changes:

• The JSON key "to" in the message expects a recipient address, i think an array would be better
  "to": "["Recipient_1", "Recipient_2",......,"Recipient_n"]"
• For RadixMessage class, "to" is of type "RadixAddress". I think it should be changed int something like
  "List toAddress" to be able to transfer the recipient addresses.
• I think it would be useful if the addReader method of the DataBuilder of the Data class would also accept
  lists as parameters. I think the same for storeData API class. Internally in the used DataStore class the
  addresses already stored in lists.