Implements tail() for DataFrame & Series

[itholic]

Implements pandas.DataFrame.tail() (https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.tail.html#pandas.DataFrame.tail) and pandas.Series.tail(https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.tail.html#pandas.Series.tail)

For DataFrame

>>> df = ks.DataFrame({'animal':['alligator', 'bee', 'falcon', 'lion',
...                    'monkey', 'parrot', 'shark', 'whale', 'zebra']})
>>> df
      animal
0  alligator
1        bee
2     falcon
3       lion
4     monkey
5     parrot
6      shark
7      whale
8      zebra

>>> df.tail(3)
  animal
8  zebra
7  whale
6  shark

For Series

>>> df = ks.DataFrame({'animal':['alligator', 'bee', 'falcon', 'lion']})
>>> df.animal.tail(2)
3      lion
2    falcon
Name: animal, dtype: object

[codecov-io]

Codecov Report

Merging #1055 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1055      +/-   ##
==========================================
+ Coverage   95.13%   95.13%   +<.01%     
==========================================
  Files          34       34              
  Lines        6765     6767       +2     
==========================================
+ Hits         6436     6438       +2     
  Misses        329      329

Impacted Files	Coverage Δ
databricks/koalas/missing/frame.py	100% <ø> (ø)	⬆️
databricks/koalas/missing/series.py	100% <ø> (ø)	⬆️
databricks/koalas/series.py	96.42% <100%> (ø)	⬆️
databricks/koalas/frame.py	96.56% <100%> (ø)	⬆️

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[HyukjinKwon]

Looks okay.

[softagram-bot]

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• Co-change Alert: You modified frame.py. Often test_dataframe.py (koalas/tests) is modified at the same time.

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[HyukjinKwon]

FYI, I plan to propose a new API tail in Spark itself. If we manage to add it, we could efficiently do that.

[HyukjinKwon]

Can we hold on for a while? We could leave a note about that if we add it in Spark.

[itholic]

@HyukjinKwon ah, that sounds great!! 👍
Okay, then just let me know for fix this after tail will be added on spark,

[HyukjinKwon]

PR submitted at apache/spark#26809. Let's see how it goes.

[itholic]

@HyukjinKwon awesome! thanks for reminding me 👍

[HyukjinKwon]

Now, Apache Spark 3.0 will have an API DataFrame.tail. Let's revisit this when Spark 3.0 is out.