Add default column value param

[kykyi]

In relation to #502, this PR (and this fork) introduces a default value using pandas.fillna()! When building Columns in a DataFrameSchema, you can add a default argument to fill NaNs in dataframe being validated:

>>> df = pd.DataFrame({"some_col": [None]})
>>> df
    some_col
0  None
>>> schema = DataFrameSchema(columns={"some_col": Column(str, default="the default value")})
>>> df = schema.validate(df)
>>> df
    some_col
0  the default value
>>> 🚀 

This PR includes the updated code, tests to assert default works as expected, and adds docstrings in line with the rest of the codebase.

TODO:

• Linting? Confirm what standards are used 💅
• Approval! Ensure this PR covers the full scope ✅

[cosmicBboy]

looks like there are some mypy errors and unit test errors.

Be sure to setup and run pre-commit and run unit tests to make sure things are running locally!

[cosmicBboy]

taking a look now @kykyi! gonna help out with the conflicts

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c9e7a6e) 97.37% compared to head (e66abbd) 97.37%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1136   +/-   ##
=======================================
  Coverage   97.37%   97.37%           
=======================================
  Files          65       65           
  Lines        4912     4916    +4     
=======================================
+ Hits         4783     4787    +4     
  Misses        129      129           

Impacted Files	Coverage Δ
pandera/api/pandas/components.py	99.11% <ø> (ø)
pandera/api/pandas/container.py	99.25% <ø> (ø)
pandera/api/pandas/model_components.py	100.00% <ø> (ø)
pandera/backends/pandas/components.py	96.66% <ø> (ø)
pandera/api/base/model_components.py	90.56% <100.00%> (+0.18%)	⬆️
pandera/api/pandas/array.py	100.00% <100.00%> (ø)
pandera/backends/pandas/array.py	98.14% <100.00%> (+0.03%)	⬆️

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[kykyi]

Thanks @cosmicBboy 👌 🙌

Anything else needed on this PR now?

[cosmicBboy]

🚀

[cosmicBboy]

Anything else needed on this PR now?

Nope, ready to merge! thanks for the contribution @kykyi , and congrats on your first PR to pandera!

[kykyi]

Great thanks for all your help 😄😄