Add tests for Kahan

[JeffFessler]

Working towards improving code coverage...

[coveralls]

Pull Request Test Coverage Report for Build 3313695501

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.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+5.5%) to 92.627%

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Totals
Change from base Build 3313520027:	5.5%
Covered Lines:	201
Relevant Lines:	217

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💛 - Coveralls

[JeffFessler]

@ivanslapnicar any comments?
And are there any other tests that would be useful to verify correctness?

[ivanslapnicar]

@JeffFessler, all changes are fine. The package is now very nice and complete.