Deltas created by parser should be aware of the source if there are multiple sources that produce the value

[fabdrol]

In the current SK spec, multiple engines reporting RPM are recorded like this:

propulsion.engine1.revolutions
propulsion.engine2.revolutions
propulsion.engine3.revolutions
etc

In the current RPM hook a delta is created with propulsion.engine1.revolutions hardcoded as the path. This is an issue, there are more than one engines. We need a way to indicate, to the hook, what source the data is coming from and how that should affect the resulting delta.

One way is to use context, and to allow the parser to be instantiated with a different context both at the global level (e.g. new Parser({ context: '...' }) and at the individual sentence-level (e.g. parser.parse(sentence, '<context>')).

[tkurki]

context is sort of loaded in Signal K, I wouldn't use that. I would go for something more sentence specific, like

new Parser({
  RPM: {
    engineId: 'starboard'
  }
})

and use sensible defaults in case there is no configuration.