ENH: rework geogr module

[bifbof]

I had the following problem:
I wanted to import Positionfixes into the file distances.py to use the class there.
But this created a circular import, positionfixes.py imports util.py which imports distances.py which now newly imports positionfixes.py, and with that python gave up.
To fix this I moved all the speed calculations out of util.py into distances.py, thus util.py does not have to import distances.py anymore. Further I have reworked the geogr module a bit.
What is part of this PR:

• moved get_speed_positionfixes and get_speed_triplegs to geogr/distances.py
• moved haversine_dist to geogr/distances.py and renamed to point_haversine_dist to make clear that is still about points, even without the ti.geogr.point_distance.haversine_dist path.
• reimported all distance functions to ti.geogr such that we now can use ti.geogr.check_gdf vs. ti.geogr.distances.check_gdf.

[bifbof]

I am sorry for these big PR. Normally I keep them under 400 LOC, on the other hand this PR only moves code around, I've not written any additional code.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (93ad546) 93.35% compared to head (d37720c) 93.35%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #537   +/-   ##
=======================================
  Coverage   93.35%   93.35%           
=======================================
  Files          33       32    -1     
  Lines        2136     2136           
  Branches      410      410           
=======================================
  Hits         1994     1994           
  Misses        129      129           
  Partials       13       13           

Files Changed	Coverage Δ
trackintel/model/util.py	100.00% <ø> (ø)
trackintel/preprocessing/staypoints.py	99.08% <ø> (ø)
trackintel/analysis/labelling.py	96.77% <100.00%> (ø)
trackintel/geogr/__init__.py	100.00% <100.00%> (ø)
trackintel/geogr/distances.py	96.66% <100.00%> (+2.16%)	⬆️
trackintel/model/positionfixes.py	100.00% <100.00%> (ø)
trackintel/model/triplegs.py	100.00% <100.00%> (ø)
trackintel/preprocessing/positionfixes.py	98.80% <100.00%> (ø)
trackintel/preprocessing/trips.py	97.50% <100.00%> (ø)

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[hongyeehh]

Thanks for the rearrangement; this makes the code more structured, especially moving the speed calculation to geogr.