Implement Index.nunique

[RainFung]

Implement Index.nunique by F.countDistinct

[codecov-io]

Codecov Report

Merging #1132 into master will decrease coverage by <.01%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #1132      +/-   ##
==========================================
- Coverage   95.18%   95.18%   -0.01%     
==========================================
  Files          35       35              
  Lines        7060     7055       -5     
==========================================
- Hits         6720     6715       -5     
  Misses        340      340

Impacted Files	Coverage Δ
databricks/koalas/series.py	96.46% <ø> (-0.05%)	⬇️
databricks/koalas/missing/indexes.py	100% <ø> (ø)	⬆️
databricks/koalas/base.py	96.33% <100%> (+0.15%)	⬆️
databricks/koalas/indexes.py	96.29% <50%> (-0.35%)	⬇️
databricks/koalas/frame.py	96.86% <0%> (+0.05%)	⬆️
databricks/koalas/indexing.py	94.57% <0%> (+0.21%)	⬆️

---

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[softagram-bot]

Softagram Impact Report for pull/1132 (head commit: f515dd9)

⚠️ Copy paste found

ℹ️ indexes.py: Copy paste fragment inside the same file on lines 762, 1136:

            raise NotImplementedError(
                \"Doesn't support symmetric_difference between Index & MultiIndex for now\")

        sdf_self = self._kdf._s...(truncated 477 chars)

ℹ️ test_indexes.py: Copy paste fragment on line 30 shared with ../test_dataframe.py, ../test_numpy_compat.py:

    @property
    def pdf(self):
        return pd.DataFrame({
            'a': [1, 2, 3, 4, 5, 6, 7, 8, 9],
            'b': [4, 5, 6, 3, 2, 1, ...(truncated 160 chars)

Now that you are on the file, it would be easier to pay back some tech. debt.

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• Permalink: Full report for pull/1132

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[HyukjinKwon]

@RainFung sorry for my late response. There's Series.nunique implementation. Can we move that implementation to IndexOpsMixin to support this in Index?

[RainFung]

OK, i will implement it based on IndexOpsMixin

[HyukjinKwon]

@RainFung can you resolve conflicts? Looks good.