implement dataframe types

[cosmicBboy]

• added submodules in pandera.typing module for dask, modin, koalas
• new documentation for mypy integration, other dataframe library support
• update copy on existing documentation - expand scope

[codecov]

Codecov Report

Merging #672 (e0fa3bb) into release/0.8.0 (6eb5b40) will decrease coverage by 0.02%.
The diff coverage is 94.81%.

@@                Coverage Diff                @@
##           release/0.8.0     #672      +/-   ##
=================================================
- Coverage          98.42%   98.40%   -0.03%     
=================================================
  Files                 34       39       +5     
  Lines               3676     3755      +79     
=================================================
+ Hits                3618     3695      +77     
- Misses                58       60       +2     

Impacted Files	Coverage Δ
pandera/io.py	100.00% <ø> (ø)
pandera/typing/pandas.py	87.17% <87.17%> (ø)
pandera/typing/dask.py	91.66% <91.66%> (ø)
pandera/external_config.py	100.00% <100.00%> (ø)
pandera/model.py	95.38% <100.00%> (+0.03%)	⬆️
pandera/typing/__init__.py	100.00% <100.00%> (ø)
pandera/typing/common.py	97.43% <100.00%> (ø)
pandera/typing/koalas.py	100.00% <100.00%> (ø)
pandera/typing/modin.py	100.00% <100.00%> (ø)
pandera/model_components.py	100.00% <0.00%> (ø)

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