Implement Index.dropna & MultiIndex.dropna (#938)

There is functionality `pandas.Index.dropna` (https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Index.dropna.html#pandas.Index.dropna)

support it also for Koalas same as pandas.

```python
>>> df = ks.DataFrame([[1, 2], [4, 5], [7, 8]],
...                   index=['cobra', 'viper', None],
...                   columns=['max_speed', 'shield'])
>>> df
       max_speed  shield
cobra          1       2
viper          4       5
NaN            7       8

>>> df.index.dropna()
Index(['cobra', 'viper'], dtype='object')

>>> df.to_pandas().index.dropna()
Index(['cobra', 'viper'], dtype='object')
```

databricks/koalas/indexes.py

@@ -18,7 +18,7 @@
 Wrappers for Indexes to behave similar to pandas Index, MultiIndex.
 """
 
-from functools import partial
+from functools import partial, reduce
 from typing import Any, List, Optional, Tuple, Union
 
 import pandas as pd
@@ -39,6 +39,7 @@
 from databricks.koalas.series import Series
 from databricks.koalas.internal import _InternalFrame
 from databricks.koalas.utils import name_like_string
+from databricks.koalas.internal import _InternalFrame
 
 
 class Index(IndexOpsMixin):
@@ -449,6 +450,61 @@ def is_object(self):
         """
         return is_object_dtype(self.dtype)
 
+    def dropna(self):
+        """
+        Return Index or MultiIndex without NA/NaN values
+
+        Examples
+        --------
+
+        >>> df = ks.DataFrame([[1, 2], [4, 5], [7, 8]],
+        ...                   index=['cobra', 'viper', None],
+        ...                   columns=['max_speed', 'shield'])
+        >>> df
+               max_speed  shield
+        cobra          1       2
+        viper          4       5
+        NaN            7       8
+
+        >>> df.index.dropna()
+        Index(['cobra', 'viper'], dtype='object')
+
+        Also support for MultiIndex
+
+        >>> midx = pd.MultiIndex([['lama', 'cow', 'falcon'],
+        ...                       [None, 'weight', 'length']],
+        ...                      [[0, 1, 1, 1, 1, 1, 2, 2, 2],
+        ...                       [0, 1, 1, 0, 1, 2, 1, 1, 2]])
+        >>> s = ks.Series([45, 200, 1.2, 30, 250, 1.5, 320, 1, None],
+        ...               index=midx)
+        >>> s
+        lama    NaN        45.0
+        cow     weight    200.0
+                weight      1.2
+                NaN        30.0
+                weight    250.0
+                length      1.5
+        falcon  weight    320.0
+                weight      1.0
+                length      NaN
+        Name: 0, dtype: float64
+
+        >>> s.index.dropna()  # doctest: +SKIP
+        MultiIndex([(   'cow', 'weight'),
+                    (   'cow', 'weight'),
+                    (   'cow', 'weight'),
+                    (   'cow', 'length'),
+                    ('falcon', 'weight'),
+                    ('falcon', 'weight'),
+                    ('falcon', 'length')],
+                   )
+        """
+        kdf = self._kdf.copy()
+        sdf = kdf._internal.sdf.select(self._internal.index_scols).dropna()
+        internal = _InternalFrame(sdf=sdf, index_map=self._internal.index_map)
+        kdf = DataFrame(internal)
+        return Index(kdf) if type(self) == Index else MultiIndex(kdf)
+
     def unique(self, level=None):
         """
         Return unique values in the index.

databricks/koalas/missing/indexes.py

@@ -52,7 +52,6 @@ class _MissingPandasLikeIndex(object):
     drop = unsupported_function('drop')
     drop_duplicates = unsupported_function('drop_duplicates')
     droplevel = unsupported_function('droplevel')
-    dropna = unsupported_function('dropna')
     duplicated = unsupported_function('duplicated')
     equals = unsupported_function('equals')
     factorize = unsupported_function('factorize')
@@ -145,7 +144,6 @@ class _MissingPandasLikeMultiIndex(object):
     drop = unsupported_function('drop')
     drop_duplicates = unsupported_function('drop_duplicates')
     droplevel = unsupported_function('droplevel')
-    dropna = unsupported_function('dropna')
     duplicated = unsupported_function('duplicated')
     equal_levels = unsupported_function('equal_levels')
     equals = unsupported_function('equals')

docs/source/reference/indexing.rst

@@ -45,6 +45,7 @@ Missing Values
 .. autosummary::
    :toctree: api/
 
+   Index.dropna
    Index.isna
    Index.notna
 
@@ -96,6 +97,13 @@ MultiIndex Properties
    MultiIndex.ndim
    MultiIndex.T
 
+MultiIndex Missing Values
+~~~~~~~~~~~~~~~~~~~~~~~~~
+.. autosummary::
+   :toctree: api/
+
+   MultiIndex.dropna
+
 MultiIndex Modifying and computations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 .. autosummary::