Dataiter

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* `GeoJSON`: New class

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* `ListOfDicts.sort`: Handle descending sort for all types

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* `ListOfDicts`: Make obsoletion a warning instead of an error

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* `DataFrame`: Fix error printing blank strings (8)

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* `DataFrame.filter`: Add `colname_value_pairs` argument
* `DataFrame.filter_out`: Add `colname_value_pairs` argument
* `ListOfDicts.__init__`: Remove arguments not intended for external use
* `ListOfDicts.rename`: Preserve order of keys
* Add documentation: https://dataiter.readthedocs.io/

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* `Vector.__init__`: Speed up by fixing type deduction