Implement Index.copy & MultiIndex.copy

[itholic]

like pandas (https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Index.copy.html#pandas.Index.copy)

implement function copy for Index

>>> df = ks.DataFrame([[1, 2], [4, 5], [7, 8]],
...                   index=['cobra', 'viper', 'sidewinder'],
...                   columns=['max_speed', 'shield'])
>>> df
            max_speed  shield
cobra               1       2
viper               4       5
sidewinder          7       8
>>> df.index
Index(['cobra', 'viper', 'sidewinder'], dtype='object')

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

>>> df.index.copy(name='snake')
Index(['cobra', 'viper', 'sidewinder'], dtype='object', name='snake')

[itholic]

While i'm here, i'm going to make this possible to support MultiIndex also.
Above resolved:

>>> midx = pd.MultiIndex([['a', 'b', 'c'],
...                       ['lama', 'cow', 'falcon'],
...                       ['speed', 'weight', 'length']],
...                      [[0, 0, 0, 0, 0, 0, 1, 1, 1],
...                       [0, 0, 0, 1, 1, 1, 2, 2, 2],
...                       [0, 0, 0, 0, 1, 2, 0, 1, 2]]
...  )
>>> s = ks.Series([45, 200, 1.2, 30, 250, 1.5, 320, 1, 0.3],
...              index=midx)

>>> s.index
MultiIndex([('a',   'lama',  'speed'),
            ('a',   'lama',  'speed'),
            ('a',   'lama',  'speed'),
            ('a',    'cow',  'speed'),
            ('a',    'cow', 'weight'),
            ('a',    'cow', 'length'),
            ('b', 'falcon',  'speed'),
            ('b', 'falcon', 'weight'),
            ('b', 'falcon', 'length')],
           )

>>> s.index.copy()
MultiIndex([('a',   'lama',  'speed'),
            ('a',   'lama',  'speed'),
            ('a',   'lama',  'speed'),
            ('a',    'cow',  'speed'),
            ('a',    'cow', 'weight'),
            ('a',    'cow', 'length'),
            ('b', 'falcon',  'speed'),
            ('b', 'falcon', 'weight'),
            ('b', 'falcon', 'length')],
           )

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[codecov-io]

Codecov Report

Merging #939 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #939      +/-   ##
==========================================
+ Coverage   94.46%   94.47%   +<.01%     
==========================================
  Files          34       34              
  Lines        6382     6390       +8     
==========================================
+ Hits         6029     6037       +8     
  Misses        353      353

Impacted Files	Coverage Δ
databricks/koalas/missing/indexes.py	100% <ø> (ø)	⬆️
databricks/koalas/indexes.py	97.38% <100%> (+0.18%)	⬆️

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[HyukjinKwon]

Maybe we should add an example but I think it's fine for now since it's incomplete for now due to missing name anyway.

[itholic]

thanks for reviewing. i'm planning to implement name, after then i'll add the example here also.