ENH: enable subclassing for tours

[bifbof]

The last class. For reference see #490.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.13% 🎉

Comparison is base (296f397) 93.31% compared to head (3923d12) 93.44%.

❗ Current head 3923d12 differs from pull request most recent head 2656087. Consider uploading reports for the commit 2656087 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #530      +/-   ##
==========================================
+ Coverage   93.31%   93.44%   +0.13%     
==========================================
  Files          34       34              
  Lines        2153     2166      +13     
  Branches      409      414       +5     
==========================================
+ Hits         2009     2024      +15     
+ Misses        130      129       -1     
+ Partials       14       13       -1     

Files Changed	Coverage Δ
trackintel/__init__.py	100.00% <100.00%> (ø)
trackintel/io/file.py	100.00% <100.00%> (ø)
trackintel/io/postgis.py	96.07% <100.00%> (ø)
trackintel/model/__init__.py	100.00% <100.00%> (ø)
trackintel/model/tours.py	100.00% <100.00%> (+10.52%)	⬆️

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[hongyeehh]

Looks great from my side, just one small docstring question.