Implement Series.mad

[90jam]

implement mad in Series

>>> s = ks.Series([1, 2, 3, 4])
>>> s.mad()
1.0

[codecov-io]

Codecov Report

Merging #1503 into master will decrease coverage by 2.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1503      +/-   ##
==========================================
- Coverage   93.93%   91.78%   -2.16%     
==========================================
  Files          36       36              
  Lines        8445     8450       +5     
==========================================
- Hits         7933     7756     -177     
- Misses        512      694     +182     

Impacted Files	Coverage Δ
databricks/koalas/missing/series.py	100.00% <ø> (ø)
databricks/koalas/series.py	96.98% <100.00%> (+0.02%)	⬆️
databricks/koalas/usage_logging/__init__.py	24.32% <0.00%> (-70.28%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	50.00% <0.00%> (-50.00%)	⬇️
databricks/koalas/__init__.py	73.07% <0.00%> (-19.24%)	⬇️
databricks/conftest.py	88.67% <0.00%> (-7.55%)	⬇️
databricks/koalas/frame.py	92.56% <0.00%> (-3.02%)	⬇️
databricks/koalas/utils.py	94.25% <0.00%> (-2.40%)	⬇️
databricks/koalas/namespace.py	84.49% <0.00%> (-1.61%)	⬇️
databricks/koalas/plot.py	93.33% <0.00%> (-0.96%)	⬇️
... and 1 more

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[itholic]

Seems good to me.

[codecov-commenter]

Codecov Report

Merging #1503 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1503   +/-   ##
=======================================
  Coverage   94.01%   94.02%           
=======================================
  Files          36       36           
  Lines        8458     8463    +5     
=======================================
+ Hits         7952     7957    +5     
  Misses        506      506           

Impacted Files	Coverage Δ
databricks/koalas/missing/series.py	100.00% <ø> (ø)
databricks/koalas/series.py	97.72% <100.00%> (+0.01%)	⬆️

---

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