Sliding stop implementation

[carolineychen8]

Added implementation in sliding.py and added tests to stop detection tests.

[carolineychen8]

Add dbscan section to visit attribution
Sliding part: https://github.com/mie-lab/trackintel/blob/master/trackintel/preprocessing/positionfixes.py#L14
Postprocessing - merge_stops to merge stops that are consecutive and in the same location (delta, T_gap, location="loc_id" -- column name). Merge_staypoints implementation, so generate locations creates the location column and thn merge_staypoints combines

[andresmondragont]

same error as #216 nomad/tests/test_io.py::test_from_df_values_types_na