Add support for RPM sentence

[tkurki]

https://www.tronico.fi/OH6NT/docs/NMEA0183.pdf

RPM Revolutions

       1 2 3   4   5 6
       | | |   |   | |
$--RPM,a,x,x.x,x.x,A*hh
1) Source; S = Shaft, E = Engine
2) Engine or shaft number
3) Speed, Revolutions per minute
4) Propeller pitch, % of maximum, "-" means astern
5) Status, A means data is valid
6) Checksum

[Shelis1976]

I'm little confused. When I use emulator and the data comes from nmea183 and it is passing thru SignalK to the TCP connections - all is ok on the QTVLM - RPM works. If I connect my Engine HAt who produce only SignalK sentence to Signal K serwer RPM on QTVLM - no works. What is wrong? I think SignalK serwer not decode from Signal K to nmea183 ... ????
Mayby there is some problem with SignalPaths. When I have path tanks.fuel.A.currentLevel - Open CPN dont recognise them. But When I put tanks.fuel.1.currentLevel all is Ok. I mean engine dashboard not DasboardSK - In this we can choose paths

[tkurki]

This issue just marks that ”somebody should implement RPM” it is not supported yet.

[ManfredRad]

I have created a new RPM sentence. However, ignoring propeller pitch, since this may not be a typical data set on pleasure boats. In addition I created XDR sentences for fuel level and eng temp and battery voltages. If interested I could raise a pull request. Or you check first my repository: https://github.com/ManfredRad/signalk-to-nmea0183