fix error in Column.pandas_dtype

[cosmicBboy]

This PR fixes the error reported in #192 where Columns had its own pandas_dtype property. This diff adds a pandas_dtype getter and setter to the SeriesSchemaBase class for consistency.

[codecov-commenter]

Codecov Report

Merging #256 into master will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
+ Coverage   96.45%   96.55%   +0.10%     
==========================================
  Files          15       15              
  Lines        1299     1308       +9     
==========================================
+ Hits         1253     1263      +10     
+ Misses         46       45       -1     

Impacted Files	Coverage Δ
pandera/schema_components.py	98.41% <100.00%> (-0.02%)	⬇️
pandera/schemas.py	96.52% <100.00%> (+0.37%)	⬆️

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