bugfix in gauss for large n

[stevengj]

Should fix #109.

Our arbitrary-precision eigensolver works by Newton's method on $\det(J - \lambda I)$ where $J$ is the Jacobi matrix, but the problem is that the determinant calculation easily underflows. However, since what we actually want is only the ratio $\det(J - \lambda I) / \det'(J - \lambda I)$ (the Newton step), we can rescale to this ratio at each step of the tridiagonal-determinant iteration to suppress under/ovrerflow.

On a separate note, the Newton iteration was missing the break statement on the convergence check, so it was taking 1000x longer than necessary!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.54%. Comparing base (d5bbb69) to head (a529373).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #111   +/-   ##
=======================================
  Coverage   98.53%   98.54%           
=======================================
  Files           7        7           
  Lines         682      686    +4     
=======================================
+ Hits          672      676    +4     
  Misses         10       10           

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