polar_training2tcx fixes and improvements

[Sterearch]

I used the tools to get training data from Polar M430 and import to Strava (thanks a lot for this project BTW!). For the result to meet my needs, I had to do the changes as in this PR.

Commit 2726621 fixes the loop break condition (line 119) to check route_i as the route samples counter. Without the change, it could stop processing the data prematurely. Also, first_sample_with_gps = false if first_sample_with_gps (line 176) is removed as this variable is not used anywhere else.

Commit 881f3d1 removes the xmlns attribute from <Track> and <Trackpoint> (lines 177, 183 in the original code). It was the key to make Strava import the tracks successfully. I also checked that TCX files created with the PolarFlow app do not contain xmlns for these elements either.

Commit e979012 uncomments and slightly improves the check for route samples to match the currently processed time stamp (line 139). Without this change, all route samples are placed at the beginning of the training, ignoring the actual time of GPS [in]availability. There seem to be some differences with how PolarFlow handles it, but I feel matching the time stamps gives more accurate results.