Adds support for NMEA 0183 MDA sentence type

[thorsteinssonh]

Adds support for NMEA 0183 MDA sentence type, containing meteorological composite.
This message type is used by Thies Clima, and probably other major instrument makers.

Changes to be committed:
modified: README.md
new file: hooks/MDA.js
modified: hooks/index.js
new file: test/MDA.js

[sbender9]

Looks good to me.

[tkurki]

Thanks! This is mergeable, but I suggest two changes.

You could make it a little bit more elegant / less verbose if you construct the values array directly as you go through the fields instead of setting variables conditionally and then later on having another null condition check on what you just set a few lines earlier.

Like this:

  const values = []
  ...
  if( parts[4] != '' ) {
    values.push({
      path: 'environment.outside.temperature',
      value: utils.transform(utils.float(parts[4]), 'c', 'k')
    })
  }
  ...
  const delta = {
    updates: [
      {
        source: tags.source,
        timestamp: tags.timestamp,
        values
      }
    ],
  }

Another nitpick would be to change != to !==, which is a good practice to do always in Javascript, even if it makes no difference here. See https://stackoverflow.com/questions/359494/which-equals-operator-vs-should-be-used-in-javascript-comparisons

[thorsteinssonh]

Thanks tkurki and sbender9.
Ok I have pushed the improvements suggested by tkurki.
Note that this MDA message type can result in duplicate outside.pressure and wind.speedOverGround, depending on how the manufacturer uses the message.

It seems best just to allow those duplicates, if the manufacturer puts that into the message, even though they should contain exact same values.

Reason for the duplicates is that MDA definition is rather clumbsy, allowing multiple wind speed and pressure entries in differen units. In signalk this just gets converted to SI units.

[tkurki]

Well, we really should have a precommit hook to run lint.

Thank you very much!