extra stopping condition

[Huangzizhou]

Extra stopping condition based on grad.dot(delta_x). A better way of adding this is to get rid of the dependency on cppopt lib but I'm trying to make minimal changes before the deadline.

Reason why this is a reasonable stopping condition:
Suppose we solve a quadratic minimization $J = \frac{1}{2}x^TAx - bx$, the minimizer is $\hat{x}=A^{-1}b$. If the newton method is used, then $\Delta x = A^{-1}(Ax-b) = x - \hat{x}$, and $\nabla J \cdot \Delta x = (Ax-b)^T(x-\hat{x})=(x-\hat{x})^TA(x-\hat{x})$, i.e. the error under $A$-norm.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (73d9800) 89.03% compared to head (cce9a19) 89.00%.

Files	Patch %	Lines
src/polysolve/nonlinear/Solver.cpp	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #68      +/-   ##
==========================================
- Coverage   89.03%   89.00%   -0.03%     
==========================================
  Files          51       51              
  Lines        1997     2001       +4     
==========================================
+ Hits         1778     1781       +3     
- Misses        219      220       +1     

Flag	Coverage Δ
polysolve	89.00% <71.42%> (-0.03%)	⬇️

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