Changing airports to EHAM throws

[n8rzz]

ref: zlsa#754

this fails in v3.1.0 due to a _comment key within the stars definition.

In v3.2.0 that has been addressed, however, this still fails from within the PositionModel.

PositionModel.calculatePosition = (coordinates, referencePostion, magneticNorth) => {
    if (!coordinates || !referencePostion || !magneticNorth) {
        throw new TypeError('Invalid parameter. PositionModel.getPosition() requires coordinates, referencePostion ' +
            'and magneticNorth as parameters');
    }

    // ...

magneticNorth is being evaluated as falsey (magneticNorth = 0) thus, it throws.

[n8rzz]

this may be fixed by future refactor #178

[n8rzz]

• add test case for failing condition:

ava('.calculatePosition() does not throw when it receives 0 for magnetic_north', t => {
    t.notThrows(() => new PositionModel(LAT_LONG_MOCK, airportPositionFixtureKLAS, 0));
    t.notThrows(() => PositionModel.calculatePosition(LAT_LONG_MOCK, airportPositionFixtureKLAS, 0));
});

• update PositionModel to handle magnetic_north of 0

[n8rzz]

looks to be the same problem within the AirspaceModel, where we are using !magneticNorth which evaluates to false when 0 passed