A pass over the TTP model flagging things with TODOs, REFINEs, and SPECs

Remaining TODOs:
* flow_tables
* group_entry_types
* packet_out
* identifiers::extensions
* parameters::default

Change-Id: I402c91608818805c2e67c0adfc3e0bb0d21cba3e
Signed-off-by: Colin Dixon <colin@colindixon.com>