Not all SignalK data is available over all interfaces

[bkp7]

Currently:

• REST API (signalk-http) MAY be implemented
• Websockets (signalk-ws) not clear whether implementation is mandatory or optional
• TCP streaming (signalk-tcp) MAY be implemented - but no documentation (schemas mention MQTT and STOMP)
• signalk over UDP is mentioned in docs but is forbidden by the schema

Currently only value/values are well defined within the Websockets interface (deltas). The Websocket get, list and put functions which allow access to all data via websockets are not well documented see #465.

Q: Is implementation of a websockets interface by a server mandatory?
Q: Is implementation of get within websockets mandatory? (presumably yes since the REST API is optional)

There is currently no way to stream meta or resource changes although PR #454 seeks to deal with meta.