Support Hessians on non-tracer outputs

[gdalle]

Fix #98 by adding the appropriate methods for converting Hessian tracers to a matrix when the output is not a tracer or dual

Add connectivity, gradient and Hessian tests for f(x) = 0

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.88%. Comparing base (f1bf915) to head (a88883f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #99      +/-   ##
==========================================
+ Coverage   79.62%   79.88%   +0.25%     
==========================================
  Files          17       17              
  Lines         702      706       +4     
==========================================
+ Hits          559      564       +5     
+ Misses        143      142       -1     

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[adrhill]

Thanks!