Dataiter

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* `DataFrame.aggregate`: Fix `UnicodeEncodeError` with string columns
* `DataFrame.unique`: Fix `UnicodeEncodeError` with string columns
* `ListOfDicts.select`: Return keys in requested order
* `Vector.__repr__`: Add custom conversion to string for display
* `Vector.__str__`: Add custom conversion to string for display
* `Vector.to_string`: Add custom conversion to string for display
* `Vector.to_strings`: Add custom conversion to string for display

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* `Array`: Rename to `Vector`
* `Vector.head`: New method
* `Vector.range`: New method
* `Vector.sample`: New method
* `Vector.sort`: New method
* `Vector.tail`: New method
* `Vector.unique`: New method

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* `DataFrame`: New class
* `ListOfDicts.__add__`: New method to support the `+` operator
* `ListOfDicts.__init__`: Rename, reorder arguments
* `ListOfDicts.__mul__`: New method to support the `*` operator
* `ListOfDicts.__repr__`: New method, format as JSON
* `ListOfDicts.__rmul__`: New method to support the `*` operator
* `ListOfDicts.__setitem__`: New method, coerce to `AttributeDict`
* `ListOfDicts.__str__`: New method, format as JSON
* `ListOfDicts.aggregate`: Speed up
* `ListOfDicts.anti_join`: New method
* `ListOfDicts.append`: New method
* `ListOfDicts.clear`: New method
* `ListOfDicts.extend`: New method
* `ListOfDicts.full_join`: New method
* `ListOfDicts.head`: New method
* `ListOfDicts.inner_join`: New method
* `ListOfDicts.insert`: New method
* `ListOfDicts.join`: Removed in favor of specific join types
* `ListOfDicts.left_join`: New method
* `ListOfDicts.pluck`: Add argument "default" to handle missing keys
* `ListOfDicts.print_`: New method
* `ListOfDicts.read_csv`: Add explicit arguments
* `ListOfDicts.read_json`: Relay arguments to `json.loads`
* `ListOfDicts.read_pickle`: New method
* `ListOfDicts.reverse`: New method
* `ListOfDicts.sample`: New method
* `ListOfDicts.semi_join`: New method
* `ListOfDicts.sort`: Change arguments to support sort direction better
* `ListOfDicts.tail`: New method
* `ListOfDicts.to_data_frame`: New method
* `ListOfDicts.to_pandas`: New method
* `ListOfDicts.unique`: Return unique by all keys if none given
* `ListOfDicts.write_csv`: Add explicit arguments
* `ListOfDicts.write_pickle`: New method

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* Make `sort` handle `None` values, sorted last

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* Fix `ObsoleteError` after multiple modifying actions

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* Add `read_csv`
* Add `read_json`
* Add `write_csv`
* Add `write_json`