`unique()` has wrong return type

cc https://github.com/databricks/koalas/issues/233

Not sure why https://github.com/databricks/koalas/pull/249 decided to return a Series instead of a numpy array… ?

>>> import findspark
>>> findspark.init()
>>> import databricks.koalas as ks
>>> import pandas as pd

>>> findspark.__version__
'1.3.0'
>>> pd.__version__
'0.24.2'
>>> ks.__version__
'0.12.0'

>>> pdf = pd.DataFrame({'a': [1, 2]})
>>> kdf = ks.DataFrame(pdf)
... (spark output removed) ...
>>> kdf['a'].unique()
0    1                                                                          
1    2
Name: a, dtype: int64
>>> pdf['a'].unique()
array([1, 2])
>>> list(pdf['a'].unique())
[1, 2]
>>> list(kdf['a'].unique())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/dist-packages/databricks/koalas/series.py", line 2407, in __getitem__
    return Series(self._scol.__getitem__(key), anchor=self._kdf, index=self._index_map)
  File "/usr/local/lib/python3.6/dist-packages/databricks/koalas/series.py", line 273, in __init__
    data=data, index=index, dtype=dtype, name=name, copy=copy, fastpath=fastpath)
  File "/usr/local/lib/python3.6/dist-packages/pandas/core/series.py", line 198, in __init__
    elif isinstance(data, (ABCSeries, ABCSparseSeries)):
  File "/usr/local/lib/python3.6/dist-packages/pandas/core/dtypes/generic.py", line 9, in _check
    return getattr(inst, attr, '_typ') in comp
  File "/opt/spark/python/pyspark/sql/column.py", line 682, in __nonzero__
    raise ValueError("Cannot convert column into bool: please use '&' for 'and', '|' for 'or', "
ValueError: Cannot convert column into bool: please use '&' for 'and', '|' for 'or', '~' for 'not' when building DataFrame boolean expressions.

>>> a = []
>>> a.extend(kdf['a'].unique())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/dist-packages/databricks/koalas/series.py", line 2407, in __getitem__
    return Series(self._scol.__getitem__(key), anchor=self._kdf, index=self._index_map)
  File "/usr/local/lib/python3.6/dist-packages/databricks/koalas/series.py", line 273, in __init__
    data=data, index=index, dtype=dtype, name=name, copy=copy, fastpath=fastpath)
  File "/usr/local/lib/python3.6/dist-packages/pandas/core/series.py", line 198, in __init__
    elif isinstance(data, (ABCSeries, ABCSparseSeries)):
  File "/usr/local/lib/python3.6/dist-packages/pandas/core/dtypes/generic.py", line 9, in _check
    return getattr(inst, attr, '_typ') in comp
  File "/opt/spark/python/pyspark/sql/column.py", line 682, in __nonzero__
    raise ValueError("Cannot convert column into bool: please use '&' for 'and', '|' for 'or', "
ValueError: Cannot convert column into bool: please use '&' for 'and', '|' for 'or', '~' for 'not' when building DataFrame boolean expressions.

>>> import numpy as np
>>> np.__version__
'1.15.2'
>>> np.array(pdf['a'].unique())
array([1, 2])
>>> np.array(kdf['a'].unique())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/dist-packages/databricks/koalas/series.py", line 2407, in __getitem__
    return Series(self._scol.__getitem__(key), anchor=self._kdf, index=self._index_map)
  File "/usr/local/lib/python3.6/dist-packages/databricks/koalas/series.py", line 273, in __init__
    data=data, index=index, dtype=dtype, name=name, copy=copy, fastpath=fastpath)
  File "/usr/local/lib/python3.6/dist-packages/pandas/core/series.py", line 198, in __init__
    elif isinstance(data, (ABCSeries, ABCSparseSeries)):
  File "/usr/local/lib/python3.6/dist-packages/pandas/core/dtypes/generic.py", line 9, in _check
    return getattr(inst, attr, '_typ') in comp
  File "/opt/spark/python/pyspark/sql/column.py", line 682, in __nonzero__
    raise ValueError("Cannot convert column into bool: please use '&' for 'and', '|' for 'or', "
ValueError: Cannot convert column into bool: please use '&' for 'and', '|' for 'or', '~' for 'not' when building DataFrame boolean expressions.

[rxin]

One addition: we should at the very least improve the error message, so people will find out about to_numpy without reading all the docs.

[rxin]

Thanks for the comment @smalory.

@ueshin can we fix the error message?