Add a basic support for the categorical type.

[ueshin]

Experimental.

Add a basic support for the categorical type.

>>> s = ks.Series(list("abbccc"), dtype="category")
>>> s
0    a
1    b
2    b
3    c
4    c
5    c
dtype: category
Categories (3, object): ['a', 'b', 'c']
>>> s.cat.categories
Index(['a', 'b', 'c'], dtype='object')
>>> s.cat.codes
0    0
1    1
2    1
3    2
4    2
5    2
dtype: int8

>>> idx = ks.CategoricalIndex(list("abbccc"))
>>> idx
CategoricalIndex(['a', 'b', 'b', 'c', 'c', 'c'],
                 categories=['a', 'b', 'c'], ordered=False, dtype='category')

>>> idx.codes
Int64Index([0, 1, 1, 2, 2, 2], dtype='int64')
>>> idx.categories
Index(['a', 'b', 'c'], dtype='object')

Currently type conversion with astype from/to the categorical type is supported, and other operations are not supported yet.

[ueshin]

This is based on #2061.

[codecov-io]

Codecov Report

Merging #2064 (acc140d) into master (557fb77) will decrease coverage by 0.05%.
The diff coverage is 91.78%.

@@            Coverage Diff             @@
##           master    #2064      +/-   ##
==========================================
- Coverage   95.27%   95.21%   -0.06%     
==========================================
  Files          57       60       +3     
  Lines       13257    13460     +203     
==========================================
+ Hits        12630    12816     +186     
- Misses        627      644      +17     

Impacted Files	Coverage Δ
databricks/koalas/categorical.py	71.05% <71.05%> (ø)
databricks/koalas/base.py	96.89% <90.62%> (-0.43%)	⬇️
databricks/koalas/indexes/category.py	91.66% <91.66%> (ø)
databricks/koalas/__init__.py	92.10% <100.00%> (+0.10%)	⬆️
databricks/koalas/indexes/base.py	97.31% <100.00%> (+0.01%)	⬆️
databricks/koalas/internal.py	96.67% <100.00%> (+0.06%)	⬆️
databricks/koalas/missing/indexes.py	100.00% <100.00%> (ø)
databricks/koalas/series.py	96.86% <100.00%> (+<0.01%)	⬆️
databricks/koalas/tests/indexes/test_base.py	100.00% <100.00%> (ø)
databricks/koalas/tests/indexes/test_category.py	100.00% <100.00%> (ø)
... and 5 more

---

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[HyukjinKwon]

Looks fine from a cursory look

[HyukjinKwon]

Maybe we could think about having a util for this codes ...

[ueshin]

Yep, I'm going to do some refactoring in the next PR.

[ueshin]

Thanks! Let me merge this now. Please feel free to leave comments.