implemented dataframe.cov

[LSturtew]

ref #1929

Implement DataFrame.cov

>>> kdf = ks.DataFrame([(1, 2), (0, 3), (2, 0), (1, 1)],
...                   columns=['dogs', 'cats'])
>>> kdf.cov()
                  dogs      cats
        dogs  0.666667 -1.000000
        cats -1.000000  1.666667

[codecov-io]

Codecov Report

Merging #2142 (7987192) into master (d7f6e88) will decrease coverage by 2.23%. The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #2142      +/-   ##
==========================================
- Coverage   95.37%   93.14%   -2.24%     
==========================================
  Files          60       60              
  Lines       13694    13601      -93     
==========================================
- Hits        13060    12668     -392     
- Misses        634      933     +299     

Impacted Files	Coverage Δ
databricks/koalas/missing/frame.py	74.57% <ø> (-25.43%)	:arrow_down:
databricks/koalas/frame.py	95.65% <85.71%> (-0.84%)	:arrow_down:
databricks/koalas/usage_logging/__init__.py	28.20% <0.00%> (-64.36%)	:arrow_down:
databricks/koalas/usage_logging/usage_logger.py	47.82% <0.00%> (-52.18%)	:arrow_down:
databricks/koalas/missing/series.py	60.56% <0.00%> (-39.44%)	:arrow_down:
databricks/koalas/__init__.py	80.26% <0.00%> (-11.85%)	:arrow_down:
databricks/koalas/missing/indexes.py	88.63% <0.00%> (-11.37%)	:arrow_down:
databricks/conftest.py	89.09% <0.00%> (-10.91%)	:arrow_down:
databricks/koalas/typedef/typehints.py	86.22% <0.00%> (-9.19%)	:arrow_down:
... and 30 more

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[xinrong-meng]

Hi @LSturtew, since Koalas has been ported to Spark as pandas API on Spark, would you like to migrate this PR to the Spark repository? Here is the ticket https://issues.apache.org/jira/browse/SPARK-36396. Otherwise, I may do that for you next week.