Pystore

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- Replace default parquet engine, deprecate Fastparquet, start using as default pyarrow
- Remove "chunksize" from `collection.py` as it's not used by dask nor pyarrow.
- Solve issue 69
- Fix collection.write by passing overwrite parameter.
- rename metadata.json to pystore_metadata.json, to avoid conflicts with pyarrow

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- Fixed deprecate 'in' operator to be compatible with pandas 1.2.0 onwards (PR 58)
- Add argument to `append()` to control duplicates (PR 57)

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- Uses `PYSTORE_PATH` environment variable, if it exists, as the path when not calling `store.set_path()` (defaults to `~/pystore`)

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- Updated PyPi install script (lib is the same as 0.1.20)

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- Fix: Resetting `config._CLIENT` to `None`

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- Fixed: Exposed set/get_partition_size and set/get_clients