altitude is not parsed

Start by tracing how TCX Trackpoint elements are parsed, using the supplied XML example as the reproduction case. Verify that AltitudeMeters is retained in the resulting GeoJSON alongside the other trackpoint data, and add or run the relevant parser test if one exists.

Should parse "AltitudeMeters" (@see example bellow).

<TrainingCenterDatabase xmlns="http://www.garmin.com/xmlschemas/TrainingCenterDatabase/v2" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.garmin.com/xmlschemas/TrainingCenterDatabase/v2 http://www.garmin.com/xmlschemas/TrainingCenterDatabasev2.xsd">
  <Activities>
    <Activity Sport="Biking">
      <Id>2012-05-28T12:09:36Z</Id>
      <Lap StartTime="2012-05-28T10:22:00Z">
        <TotalTimeSeconds>16310.0</TotalTimeSeconds>
        <DistanceMeters>102970.0</DistanceMeters>
        <Calories>3460</Calories>
        <Intensity>Active</Intensity>
        <TriggerMethod>Manual</TriggerMethod>
        <Track>
          <Trackpoint>
            <Time>2012-05-28T10:22:19Z</Time>
            <Position>
              <LatitudeDegrees>43.604123</LatitudeDegrees>
              <LongitudeDegrees>1.412692</LongitudeDegrees>
            </Position>
          </Trackpoint>
          <Trackpoint>
            <Time>2012-05-28T10:22:31Z</Time>
            <Position>
              <LatitudeDegrees>43.604123</LatitudeDegrees>
              <LongitudeDegrees>1.412692</LongitudeDegrees>
            </Position>
            <AltitudeMeters>193.1</AltitudeMeters>
          </Trackpoint>