Numerical derivatives are confusing...

[denehoffman]

Fixed a lot of small things that were probably causing incorrect Function::hessian calls.

• Finite difference step size is now strictly positive (wasn't before but the signs cancelled out) and equal to eps * (1 + |x|)
• Bounds/lack thereof were improperly mixed around in the various methods' Algorithm::postprocessing calls
• Hessian was reworked
• Other quality of life improvements

This should all lead to more-correct parameter uncertainties

[codecov]

Codecov Report

Attention: Patch coverage is 68.75000% with 15 lines in your changes missing coverage. Please review.

Project coverage is 71.16%. Comparing base (1166f06) to head (4439601).
Report is 37 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lib.rs	58.33%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            main      #31       +/-   ##
==========================================
+ Coverage   0.19%   71.16%   +70.97%     
==========================================
  Files          8        8               
  Lines       1020     1762      +742     
==========================================
+ Hits           2     1254     +1252     
+ Misses      1018      508      -510     

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