Use a different criterion for the BFGS curvature test

[sblauth]

Now, the criterion is an absolute one, in contrast to the previous implementation, where a relative one was used.
If a relative criterion is desired, damped BFGS should be chosen.

[codecov]

Codecov Report

Merging #263 (82902b7) into main (1903c87) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #263   +/-   ##
=======================================
  Coverage   92.59%   92.59%           
=======================================
  Files          83       83           
  Lines        6450     6450           
=======================================
  Hits         5972     5972           
  Misses        478      478           

Impacted Files	Coverage Δ
...cs/_optimization/optimization_algorithms/l_bfgs.py	99.39% <100.00%> (ø)

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