Enabled poly ring caching in charpoly/minpoly (when no poly ring passed in)

[JohnAAbbott]

After discussion in triage meeting today: now characteristic_polynomial(M) == characteristic_polynomial(M) works, and similarly for minpoly(M). The automatically created polynomial ring is cached. I also commented why caching is enabled.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.34%. Comparing base (956a964) to head (ef1c947).
Report is 2 commits behind head on master.

Additional details and impacted files

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  Coverage   88.34%   88.34%           
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  Files         124      124           
  Lines       31404    31404           
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  Hits        27743    27743           
  Misses       3661     3661           

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[thofma]

I am sympathetic to this change, but it would mean fixing all charpoly/characteristic_polynomial calls with a single argument in Nemo/Hecke/Oscar (functions are not allowed to create cached objects).

It becomes also be a bit unergonomic to create characteristic polynomials without caching the parent of the polynomial. I wonder whether something like charpoly(M; cached::Bool = true) would make sense. It sounds a bit ambiguous though (is it caching the characteristic polynomial?!).

Happy to hear what others think.