Gtfstk

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- Bugfixed ``feed.copy`` so that the ``dist_units_in`` of the copy equals ``dist_units_out`` of the original.
- Added some more distance sanity checks to ``calculator.compute_trip_stats`` and ``calculator.append_dist_to_stop_times``.

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- Improved ``cleaner.clean_route_short_names``.
- Removed ``utilities.clean_series``.
- Improved ``cleaner.aggregate_routes``.
- Removed some unnecessary print statements.

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- Deleted an extraneous print statement in ``calculator.create_shapes``.
- Added ``utilities.is_not_null``.
- Changed ``calculator.shapes_to_geojson`` to return a dictionary instead of a string.
- Upgraded to Pandas 0.18.1 and fixed ``calculator.downsample`` accordingly
- Added ``cleaner.aggregate_routes``.

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- Bugfix: formatted ``parent_station`` as a string in ``constants.DTYPE``.

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- Changed signature and behavior of ``create_shapes``.
- Added duplicate route short name count to ``assess``.
- Changed the behavior of ``clean_route_short_names``.
- Changed ``INT_COLS`` to ``INT_COLUMNS``.
- Moved some functions.
- Added some functions, such as a function to copy feeds.

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- Added more functions to ``calculator.py``, some of which are optional and depend on GeoPandas.
- Documented more.
- Made ``read_gtfs`` raise a more helpful error when an input path does not exist.