Bug fix: Assure that feature dataframe indices are unique

[JuliaKukulies]

The feature detection does currently not work when min_distance is not 0. An exception is raised in filter_min_distance() , because the input feature dataframe for this function can contain index values that are double-defined as a consequence of concatenating dataframes in feature_detection_multithreshold_timestep(). This PR solves #184 by assuring that the feature dataframe that is created in feature_detection_multithreshold_timestep() gets unique indices.

I also added a test for filter_min_distance() to test if features are removed as expected within a specified minimum distance and I removed redundant imports from test_feature_detection.py.

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[codecov]

Codecov Report

Base: 35.06% // Head: 35.68% // Increases project coverage by +0.61% 🎉

Coverage data is based on head (8f9ae47) compared to base (dfe3b38).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 8f9ae47 differs from pull request most recent head 7291228. Consider uploading reports for the commit 7291228 to get more accurate results

Additional details and impacted files

@@              Coverage Diff              @@
##           RC_v1.4.0     #187      +/-   ##
=============================================
+ Coverage      35.06%   35.68%   +0.61%     
=============================================
  Files             10       10              
  Lines           2099     2099              
=============================================
+ Hits             736      749      +13     
+ Misses          1363     1350      -13     

Flag	Coverage Δ
unittests	35.68% <100.00%> (+0.61%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tobac/feature_detection.py	77.90% <100.00%> (+7.18%)	⬆️

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[freemansw1]

Great work and glad that we can quickly address #184!

[freemansw1]

needs closing ```

[snilsn]

Good job @JuliaKukulies ! Nothing to complain about in this quick bug fix and great that the code coverage is increased at the same time!

[JuliaKukulies]

Thanks for the quick review! I will merge after #186 has been merged