Dataiter

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* `DataFrame.modify`: Fix grouped modify on unsorted data frame

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* `Vector.map`: Add `dtype` argument

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* `ListOfDicts.to_data_frame`: Add `strings_as_object` argument

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* `read_csv`, `read_geojson`, `DataFrame.from_pandas`,
`DataFrame.read_csv`, `GeoJSON.read`: Add `strings_as_object` argument

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* `DataFrame.slice_off`: New method
* `GeoJSON.to_data_frame`: New method
* Fix error with new column placeholder attributes in conjunction with
pop, popitem and clear

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* `DataFrame`: Add placeholder attributes for columns so that
tab completion of columns as attributes at a shell works
* `dt.from_string`: New function
* `dt.to_string`: New function
* `nrow`: Remove deprecated aggregation function
* Don't use Numba for aggregation involving strings due to bad performance