ENH: Add gpx reader for single user

[bifbof]

That was a lot simpler than I thought.
Currently we assign to all positionfixes user_id = 0.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bd83b61) 93.10% compared to head (d6bccca) 93.15%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #570      +/-   ##
==========================================
+ Coverage   93.10%   93.15%   +0.05%     
==========================================
  Files          33       33              
  Lines        2088     2105      +17     
  Branches      373      374       +1     
==========================================
+ Hits         1944     1961      +17     
  Misses        130      130              
  Partials       14       14              

Files	Coverage Δ
trackintel/io/__init__.py	100.00% <100.00%> (ø)
trackintel/io/dataset_reader.py	50.41% <100.00%> (+3.51%)	⬆️

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[NinaWie]

Looks good to me. Have you already tested whether the code can be used to generate staypoints, trips etc, using the test data from Strava and from Henry? We should test that before the workshop

[bifbof]

It can generate them for Strava (I haven't tried Henry's) and at first glance the staypoints and triplegs seem fine.