Single group implemented
Next:
- automatically create new group when GROUP_SIZE is reached
- implement (merge/)split procedure

Format for Join Notifs/Acks has been changed: they now contain a group id.
- for Join Notif, will represent the group the new node is joining
- for Join Ack, will represent the group the answering node belongs to

Network now has a map associating group ID with a Group, and two maps associating group ID with predecessors and successors in the corresponding group rings or channel rings
changed Message display method