ENH: add doc decorator

[bifbof]

Moved the introduction of @doc out of #538 to make the change more reviewable.
@doc is a decorator to take docstring templates, concatenate them and perform string substitution on them.

[codecov]

Codecov Report

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

Comparison is base (9dd980e) 93.35% compared to head (74e2a3c) 93.41%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #539      +/-   ##
==========================================
+ Coverage   93.35%   93.41%   +0.06%     
==========================================
  Files          32       32              
  Lines        2136     2156      +20     
  Branches      410      429      +19     
==========================================
+ Hits         1994     2014      +20     
  Misses        129      129              
  Partials       13       13              

Files Changed	Coverage Δ
trackintel/io/postgis.py	96.07% <100.00%> (ø)
trackintel/model/util.py	100.00% <100.00%> (ø)

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[hongyeehh]

Thanks for introducing the new doc() function. I am just still a bit confused about the import problem.