ENH: remove caching in DataFrame accessors

[bifbof]

We agreed to remove caching for DataFrame accessors to allow subtyping, as suggested in #490.
This creates overhead every time we call .as_xyz, but also provides more security since the model is now truly assured every time we use it this way.
It was necessary to adjust some tests so that they do not violate the trackintel models.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.06% 🎉

Comparison is base (2371b04) 92.47% compared to head (443644a) 92.53%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #505      +/-   ##
==========================================
+ Coverage   92.47%   92.53%   +0.06%     
==========================================
  Files          39       39              
  Lines        2019     2036      +17     
  Branches      344      346       +2     
==========================================
+ Hits         1867     1884      +17     
  Misses        133      133              
  Partials       19       19              

Files Changed	Coverage Δ
trackintel/model/locations.py	100.00% <100.00%> (ø)
trackintel/model/positionfixes.py	100.00% <100.00%> (ø)
trackintel/model/staypoints.py	100.00% <100.00%> (ø)
trackintel/model/tours.py	90.00% <100.00%> (+0.52%)	⬆️
trackintel/model/triplegs.py	100.00% <100.00%> (ø)
trackintel/model/trips.py	100.00% <100.00%> (ø)
trackintel/model/util.py	100.00% <100.00%> (ø)

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[hongyeehh]

Nice and neat, thanks!