feature/lazy-validation

[cosmicBboy]

Fixes #122

This PR adds lazy validation to schema and schema component classes. This keyword argument is passed into the validate method and is False by default. If True, the method will collect all SchemaError object that are raised during validation and raise a SchemaErrors exception.

• SchemaError objects now contain the validated data, check object, and failure cases that gave rise the error.
• SchemaErrors objects contain the collect schema errors raised during validation.

[codecov-io]

Codecov Report

Merging #207 into master will decrease coverage by 0.16%.
The diff coverage is 98.19%.

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
- Coverage   96.58%   96.42%   -0.17%     
==========================================
  Files          12       13       +1     
  Lines         996     1118     +122     
==========================================
+ Hits          962     1078     +116     
- Misses         34       40       +6     

Impacted Files	Coverage Δ
pandera/hypotheses.py	86.79% <33.33%> (ø)
pandera/schemas.py	96.05% <98.66%> (-1.05%)	⬇️
pandera/checks.py	98.70% <100.00%> (ø)
pandera/decorators.py	90.90% <100.00%> (ø)
pandera/error_formatters.py	95.00% <100.00%> (-5.00%)	⬇️
pandera/error_handlers.py	100.00% <100.00%> (ø)
pandera/errors.py	100.00% <100.00%> (ø)
pandera/schema_components.py	98.40% <100.00%> (+0.13%)	⬆️

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