CLN: fix deprecation warnings for .idxmax and .loc[]

[bifbof]

For idxmax two things got deprecated.

1. the axis keyword, this was an easy fix
2. support of columns with only NaT, we relied on this behavior. Thus I needed to rewrite code to handle this.

Further deprecated is df.loc[idx, "col"] = "string"; it creates a new column that defaults to float64, that type then gets upcasted to object when "string" gets filled into the dataframe. To avoid this we have to create the column beforehand with df["col"] = None.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (d835098) 93.05% compared to head (55a3baf) 93.07%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #526      +/-   ##
==========================================
+ Coverage   93.05%   93.07%   +0.01%     
==========================================
  Files          34       34              
  Lines        2074     2079       +5     
  Branches      388      389       +1     
==========================================
+ Hits         1930     1935       +5     
  Misses        130      130              
  Partials       14       14              

Files Changed	Coverage Δ
trackintel/analysis/location_identification.py	99.09% <100.00%> (+0.04%)	⬆️

... and 1 file with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[hongyeehh]

This is ready for merging from my side. I only have one clarification issue left. After that, you can merge this PR.