Utility classes for dealing with Garmin Forerunner data.

Supported formats:

• tcx (From Garmin Training Center)
• gpx (From LoadMyTracks)

1. DONE
   g = Garmin.open(‘file.tcx’)
   g.activities
   g.activity(‘2009-01-12T19:28:18Z’)
   activity.date
   activity.sport
   activity.distance
   activity.laps
   lap.distance, etc
   lap.trackpoints
   lap.track_points⁰.latitude
   lap.track_points⁰.longitude
   lap.track_points⁰.altitude
   lap.track_points⁰.heart_rate
   lap.track_points⁰.time
   lap.track_points⁰.distance

1. TODO
   activity.average_pace
   activity.average_heart_rate