Always include the m value of Coordinate3D in JSON

[trasch]

The m value of Coordinate3D will now always be includeded in the JSON dump of a coordinate. This means that a null value will be added for the altitude if the coordinate's altitude is nil.

let coordinateM = Coordinate3D(latitude: 15.0, longitude: 10.0, altitude: nil, m: 1234)
let coordinateDataM = try JSONEncoder().encode(coordinateM)
print(String(data: coordinateDataM, encoding: .utf8)) // [10,15,null,1234]