add exceptional shift

[Toucan4Life]

Summary of Changes

This PR fixes some cases where the search for eigenvalue could hang. This was solved by using the "Exceptional Shift" method in Wilkinson, J. H., & Reinsch, C. (1971).
Handbook for automatic computation (Vol. 2: Linear algebra).

Details

I first checked why the Lapack algorithm would converge and not hang. I looked at dlahqr procedure (https://netlib.org/lapack/explore-html/d8/df4/group__lahqr_ga100049ab080cef756846702a33e8e28f.html) saw the mention of the exceptional shift and the Ahues & Tisseur stopping criterion.

The last one was a red herring, I implemented it but the test matrix from #611 would still hang. I then implemented the exceptional shift and it worked. I tried tracing back the magical value (0.75 & -0.4375), and discovered https://www.netlib.org/eispack/hqr.f that led me to the original source:

Wilkinson, J. H., & Reinsch, C. (1971).
Handbook for automatic computation (Vol. 2: Linear algebra).

You can view the orignal magical number and algorithm implementation here : https://www.scribd.com/document/568239788/Wilkinson-Reinsch1971-Book-HandbookForAutomaticComputatio#page=373

AI was used to find the original paper

Testing

I implemented a test with the matrix from #611 that now pass successfully

Fixes #611

[Toucan4Life]

@alexhroom still one case of eigen value search hanging that should be solved now :)

[alexhroom]

thanks for this! just a couple requests for more comments...

[Toucan4Life]

Just added more comments, my bad I always forgot them. I will not resolve the conversations, I'll let you be the judge if they are good enough

[alexhroom]

looks good to me! thanks for making the changes.