Dataiter

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* `mode`: Fix to return first in case of ties (requires Python >= 3.8)
* `std`, `var`: Add `ddof` argument (defaults to 0 on account of Numba limitations)
* Don't try to dropna for non-float vectors in aggregation functions

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* Add shorthand helper functions for use with `DataFrame.aggregate`,
optionally using Numba JIT-compiled code for speed
- https://dataiter.readthedocs.io/en/latest/aggregation.html
- https://dataiter.readthedocs.io/en/latest/data-frame.html#dataiter.DataFrame.aggregate
- https://dataiter.readthedocs.io/en/latest/dataiter.html
* `DataFrame.map`: New method
* `ncol`: Removed
* `nrow`: Deprecated in favor of `dataiter.count`
* `read_csv`: New alias for `DataFrame.read_csv`
* `read_geojson`: New alias for `GeoJSON.read`
* `read_json`: New alias for `ListOfDicts.read_json`
* `read_npz`: New alias for `DataFrame.read_npz`

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* `DataFrame`: Make object columns work in various operations
* `DataFrame.from_json`: Add arguments `columns` and `dtypes`
* `DataFrame.from_pandas`: Add argument `dtypes`
* `DataFrame.full_join`: Speed up
* `DataFrame.read_csv`: Add argument `dtypes`
* `DataFrame.read_json`: Add arguments `columns` and `dtypes`
* `GeoJSON.read`: Add arguments `columns` and `dtypes`
* `ListOfDicts.fill_missing`: New method
* `ListOfDicts.from_json`: Add arguments `keys` and `types`
* `ListOfDicts.full_join`: Speed up
* `ListOfDicts.read_csv`: Add argument `types`, rename `columns` to `keys`
* `ListOfDicts.read_json`: Add arguments `keys` and `types`

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* `DataFrame`: Fix error message when column not found
* `DataFrame.aggregate`: Speed up
* `DataFrame.full_join`: Fix to join all possible columns
* `DataFrame.read_csv`: Try to avoid mixed types
* `ListOfDicts.full_join`: Fix to join all possible keys
* `ListOfDicts.write_csv`: Use minimal quoting
* `Vector.get_memory_use`: New method
* `Vector.rank`: Rewrite, add `method` argument
* `*.read_*`: Rename `fname` argument `path`
* `*.write_*`: Rename `fname` argument `path`
* Add comparison table dplyr vs. Dataiter vs. Pandas to documentation:
<https://dataiter.readthedocs.io/en/latest/comparison.html>

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* `DataFrame.read_npz`: New method to read NumPy npz format
* `DataFrame.write_npz`: New method to write NumPy npz format
* `*.read_*`: Decompress `.bz2|.gz|.xz` automatically
* `*.write_*`: Compress `.bz2|.gz|.xz` automatically

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* `DataFrame.print_missing_counts`: Fix when nothing missing
* `Vector.replace_missing`: New method