Gtfstk

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- Added Python 3.7 support (by simply updating the Travis configuration).

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- Simplified and made more robust the function ``compute_center``.
- Simplified the functions ``compute_stop_stats``, ``compute_route_stats``, and ``compute_feed_stats``.
- Added the function ``get_week``, a generalization of the function ``get_first_week``.
- Breaking change: renamed the function ``restrict_dates`` to ``subset_dates``, so as not to be confused with the function ``restrict_to_dates``. Also changed the signature of ``restrict_dates``.

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- Bugfixed warning about stops with no stop times in the function ``check_stops``.
- Bugfixed removing stations in the ``restrict_to_*`` functions.

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- Bugfixed parent station cleaning in the function ``drop_zombies``.

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- Checked for undefined parent stations in the module ``validators.py`` and added functionality to remove them in the module ``cleaners.py``.

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- Added ``split_route_types`` flag to the functions ``compute_feed_stats`` and ``compute_feed_time_series``.
- Added helper functions ``unstack_time_series`` and ``restack_time_series``.
- Named time series axis ``"datetime"``.
- Bugfixed Feed instance printing.
- Replaced NaN service speeds in time series with service distance (0 or NaN).
- Bugfixed function ``downsample`` to preserve all-NaN sections.