ENH: enable subclassing for Staypoints

[bifbof]

Here comes the next class :)
For more info see #490 .

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05% 🎉

Comparison is base (e2bc973) 92.71% compared to head (eac46d7) 92.76%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #509      +/-   ##
==========================================
+ Coverage   92.71%   92.76%   +0.05%     
==========================================
  Files          40       40              
  Lines        2086     2102      +16     
  Branches      374      381       +7     
==========================================
+ Hits         1934     1950      +16     
  Misses        133      133              
  Partials       19       19              

Files Changed	Coverage Δ
trackintel/__init__.py	100.00% <100.00%> (ø)
trackintel/analysis/tracking_quality.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/staypoints.py	100.00% <100.00%> (ø)

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