DatasetConstraints serialization and chi square calculation fixes

[pecop2]

Description

Fixing a serializing bug for DatasetConstraints that contained MultiColumnConstraints and updating the tests.
Fix for Chi-Squared statistic calculation.

General Checklist

• Tests added for this feature/bug
  if it was a bug, test must cover it.
• Conform by the style guides, by using formatter
• Documentation updated
• (optional) Please add a label to your PR

[coveralls]

Pull Request Test Coverage Report for Build 1841873043

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 21 of 24 (87.5%) changed or added relevant lines in 3 files are covered.
• 9 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.07%) to 82.601%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/whylogs/core/summaryconverters.py	7	10	70.0%

Files with Coverage Reduction	New Missed Lines	%
src/whylogs/proto/constraints_pb2.py	9	67.33%

Totals
Change from base Build 1824832732:	0.07%
Covered Lines:	4745
Relevant Lines:	5596

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💛 - Coveralls

[jamie256]

Looks good, thanks for making the fixes and refactor.