fix: solve and solve_dense respect roots in problem

[martinjrobins]

fixes #221

[codecov]

Codecov Report

❌ Patch coverage is 77.41935% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.18%. Comparing base (88c5654) to head (9a731ab).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
diffsol/src/ode_solver/method.rs	76.00%	12 Missing ⚠️
...src/ode_equations/test_models/exponential_decay.rs	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #231      +/-   ##
==========================================
+ Coverage   74.16%   74.18%   +0.01%     
==========================================
  Files          85       85              
  Lines        9309     9362      +53     
==========================================
+ Hits         6904     6945      +41     
- Misses       2405     2417      +12     

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