Add .tcx versions of previously added .fit files

These files are the .tcx files downloaded from Garmin Connect after
having uploaded the rides/hides that correspond to the previously
added .fit files.
Rename [Ff]it* to [Aa]ct* to prepare for parsing more than just .fit files
Refactor ActSample to store lat/lon in redians instead of "semicircles"

Also rewrite the geographic distance calculations in ActSampleDiff for
clarity.
Split .fit parsing out of act_sample.py into a separate fit_parser.py
Remove unused and ugly method