Dataiter

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* `dt`: New module for dealing with dates and datetimes

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* `DataFrame.from_pandas`: Speed up by avoiding unnecessary conversions
* `DataFrame.full_join`: Fix join and output when `by` is a tuple
* `GeoJSON`: Fix printing object

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* `Vector`: Handle timedeltas correctly for NA checks and printing
* `Vector.is_timedelta`: New method

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* `DataFrame.sort`: Convert object to string for sorting
* `Vector.sort`: Convert object to string for sorting
* Fix conditional Numba use when importing the numba package works,
but caching doesn't
* Add `di-open` cli command (currently not part of the default install,
but can be installed from source using `make install-cli`)

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* `DataFrame.modify`: Add support for grouped modification (19)
* `DataFrame.split`: New method
* `ListOfDicts.split`: New method

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* `DataFrame.compare`: New experimental method
* `Vector.as_string`: Add `length` argument
* Change the documentation to default to the latest release ("stable")
instead of the development version ("latest")