Add type annotations to GroupBy objects

[xinrong-meng]

#type: ignore's in b20a3b1 are mostly because mypy cannot identify _attributes of self.

We are considering adding abstract methods to GroupBy to help mypy identify. However, this should be a separate task.

[codecov-io]

Codecov Report

Merging #1889 into master will decrease coverage by 2.84%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1889      +/-   ##
==========================================
- Coverage   94.19%   91.34%   -2.85%     
==========================================
  Files          40       40              
  Lines        9939     9858      -81     
==========================================
- Hits         9362     9005     -357     
- Misses        577      853     +276     

Impacted Files	Coverage Δ
databricks/koalas/frame.py	93.40% <100.00%> (-3.33%)	⬇️
databricks/koalas/groupby.py	90.96% <100.00%> (-0.23%)	⬇️
databricks/koalas/series.py	95.68% <100.00%> (-1.29%)	⬇️
databricks/koalas/usage_logging/__init__.py	25.66% <0.00%> (-66.65%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	47.82% <0.00%> (-52.18%)	⬇️
databricks/koalas/__init__.py	76.66% <0.00%> (-13.34%)	⬇️
databricks/conftest.py	88.13% <0.00%> (-11.87%)	⬇️
databricks/koalas/spark/functions.py	88.88% <0.00%> (-7.41%)	⬇️
databricks/koalas/accessors.py	86.06% <0.00%> (-6.97%)	⬇️
databricks/koalas/namespace.py	78.45% <0.00%> (-4.85%)	⬇️
... and 16 more

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[ueshin]

LGTM, pending tests.