Pystore

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- Reversed `list_snapshots()` behaviour
- Added `collection.threaded_write(...)` method
- `collection.items` being updated using `items.add()` and an async/threaded directory read

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- Switched from `dtype_str` to `str(dtype)` (Pandas 0.25+ compatibility)
- Implemented `collection.items` and `collection.snapshots` as `property` to reduce initialization overhead
- `collection.items` and `collection.snapshots` are now of type `set()`
- Option to specify both `npartitions` and `chunksize` in `collection.append()`

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- Fixed issues 13 and 15

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- Added `pystore.read-csv()` to quickly load csv as dask dataframe, ready for storage

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- Using `os.path.expanduser("~")` to determine user's home directory
- `collection.write(...)` accepts Dask dataframes

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- Misc improvements