feature/koalas-beta

[cosmicBboy]

Addresses one part of #601

This PR introduces support for koalas object validation (DataFrame, Series, and Index), so pandera can be used like so:

Install

pip install pandera[koalas]

Then validate away!

schema = pa.DataFrameSchema(
    {
        "int_column": pa.Column(int, pa.Check.ge(0)),
        "float_column": pa.Column(float, pa.Check.le(0)),
        "str_column": pa.Column(str, pa.Check.isin(list("abcde"))),
    },
    coerce=True,
)
kdf = ks.DataFrame(
    {
        "int_column": range(10),
        "float_column": [float(-x) for x in range(10)],
        "str_column": list("aabbcceedd"),
    }
)

Notes:

• check_utils module adds a bunch of methods for checking the type of a object-to-validate. This should probably be moved somewhere else.
• littered about in the core schema validation logic are checks for koalas objects, e.g. here and here. Will clean up this tech debt as part of Abstract out validation logic to support non-pandas dataframes, e.g. spark, dask, etc #381
• there are a few places that use a footgun, i.e. enable koalas features that negatively impacts performance, e.g. here which enables the koalas compute.ops_on_diff_frames config, which allows for computations across dataframes (which can involve expensive join operations). Clearing this tech debt would require someone more familiar with the koalas project.
• This conditional checks for presence of a pandera accessor class. Need to find a way around this for modin, which doesn't currently support the accessor extension utility. Koalas does, though, so will need to implement that extension in a future PR.

This feature is in beta, so many bugs are expected.

[codecov]

Codecov Report

Merging #651 (9ed4496) into dev (c786f67) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #651      +/-   ##
==========================================
+ Coverage   98.85%   98.94%   +0.08%     
==========================================
  Files          30       31       +1     
  Lines        3398     3497      +99     
==========================================
+ Hits         3359     3460     +101     
+ Misses         39       37       -2     

Impacted Files	Coverage Δ
pandera/model.py	100.00% <ø> (ø)
pandera/__init__.py	100.00% <100.00%> (ø)
pandera/check_utils.py	100.00% <100.00%> (ø)
pandera/checks.py	98.50% <100.00%> (ø)
pandera/engines/pandas_engine.py	99.32% <100.00%> (+0.02%)	⬆️
pandera/engines/utils.py	100.00% <100.00%> (ø)
pandera/error_formatters.py	92.00% <100.00%> (-3.46%)	⬇️
pandera/errors.py	100.00% <100.00%> (ø)
pandera/external_config.py	100.00% <100.00%> (ø)
pandera/schema_components.py	99.54% <100.00%> (+0.02%)	⬆️
... and 6 more

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