Clean up logger and reduce the amount of debug messages printed

[clinssen]

Clean up logger and reduce the amount of debug messages printed. Also disable matplotlib message logging in the tests because of the enormous amounts of spam that produces. Clean up importing matplotlib across all tests.

Add new options related to alternative expM to documentation and properly pass the use_alternative_expM configuration parameter. The latter revealed that the alternative expM hangs on TestAnalyticSolverIntegration and as such has been disabled for that test.

Add a new global option disable_singularity_mitigation that keeps singularity detection (and issuing warnings for it) enabled, but disables generating alternative solvers, which can be very computationally costly.

Create debug_utils.py because this functionality is also used inside the package itself, and thus needs to be inside the odetoolbox directory rather than the tests directory.

[codecov]

Codecov Report

❌ Patch coverage is 72.27723% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.22%. Comparing base (c624f2d) to head (05acd83).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
odetoolbox/__init__.py	78.78%	7 Missing ⚠️
odetoolbox/mixed_integrator.py	40.00%	6 Missing ⚠️
ode_analyzer.py	0.00%	5 Missing ⚠️
odetoolbox/stiffness.py	0.00%	5 Missing ⚠️
odetoolbox/debug_utils.py	60.00%	2 Missing ⚠️
odetoolbox/dependency_graph_plotter.py	0.00%	1 Missing ⚠️
odetoolbox/sympy_helpers.py	66.66%	1 Missing ⚠️
odetoolbox/system_of_shapes.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #99      +/-   ##
==========================================
+ Coverage   62.53%   66.22%   +3.69%     
==========================================
  Files          15       15              
  Lines        1500     1741     +241     
==========================================
+ Hits          938     1153     +215     
- Misses        562      588      +26     

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