Gtfstk

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- Added ``map_trips`` which works like ``map_routes``.

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- Changed ``route_to_geojson`` to return LineStrings instead of a MultiLineString and added a date keyword argurment.
- Changed ``shapes_to_geojson`` to accept an optional list of shape IDs to restrict to.
- Added ``map_routes`` function to draw routes and their stops on a Folium map, if Folium is installed.
- Inserted stars in function signatures to separate boolean keyword arguments. Is this a breaking change? I say no, but it's debatable.
- Changed ``compute_trip_stats`` to accept an optional list of route IDs to restrict to.
- Clarified the doctstrings of ``compute_route_stats`` and ``compute_route_time_series`` to note that those functions can accept slices of trip stats.
- Changed ``compute_stop_stats`` and ``compute_stop_time_series`` to accept an optional list of stop IDs.

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- Stopped ``drop_zombies`` from dropping stops with location type 1 or 2.
- Changed ``CRS_WGS84`` to ``WGS84`` and removed the ``no_defs`` key to agree with GeoPandas's WGS84 CRS.
- Replaced some ``None`` outputs with empty dictionary outputs where appropriate, e.g. in ``build_shape_by_geometry``.

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- Bugfixed the ``get_dates()`` function. It was throwing an error when the calendar or calendar_dates table was empty.

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- Bugfixed the stats and time series functions. They were throwing errors in the edge case where all the given dates had no active trips.
- Bugfixed ``combine_time_series()``. Its direction ID column names were ``'0'`` and ``'1'`` but should be ``0`` and ``1``.

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- Added informative printing for Feeds.
- Removed the ``time_it`` decorator in favor of IPython's ``%time`` magic .
- Inspired by the `Transitland Dispatcher <https://transit.land/dispatcher/feed-versions/eb0cbe5ab41c9cfde0ebae42471ab5b3f712b008>`_, added the ``summarize`` function and the ``list_gtfs`` function.
- Extended several functions to accept date lists, a breaking change for the outputs of those functions. For example, now you can compute feed stats for the entire feed period more easily and quickly (by memoizing active trip IDs) than computing the stats separately for each date.
- By popular demand, redefined the ``num_trips`` indicator in route and feed time series to be the number of unique trips active in a time bin instead of the time weighted average thereof.
- Removed columns from empty DataFrames returned by ``compute_route_stats`` etc.
- Elaborated docstrings.