fix: provide a null delta when timeRemaining changes to unknown

[sbender9]

This includes the state changes from #72

I am tempted to change n2kMapper to do this null delta for all fields in all PGNs. I think that makes sense, then each pgn would not have to have code for checking state like this.

That would mean keeping state for every single value in n2k.

[tkurki]

console.log

[tkurki]

So the idea of this is to send one delta with null value when the PGN no longer has the field Time Remaining.

I am not absolutely sure if this is what we should be doing.

As discussed in SignalK/specification#93 there is a case for a client receiving deltas continuously with null values to indicate that the system is operating properly end to end but for some reason there is no valid value available at this time.

The stopgap that makes the current implementation ignore PGNs that do not have the expected field is https://github.com/SignalK/n2k-signalk/blob/master/n2kMapper.js#L90. If we change that to send null values instead it changes the behavior somewhat dramatically. This would allow for example a gauge that shows timeremaining based on deltas to indicate differently the total absence of input data and not having a valid value.

[sbender9]

So do you want to do anything with this now. I'm ok with tabling it until later. WilhelmSK ends up showing red for the "freshness" indicator anyway.

[sbender9]

Closing for alternate implementation #77