Add test for inhomogeneous term that during numerical integration is equal to zero

[clinssen]

If the dynamics of the system is defined such that there is an inhomogeneous (constant-term) component in an ODE, but if numerically, if the inhomogeneous part is zero (this could happen in the case that one or more parameters get set to zero), then because of the way the update expressions are formulated, you could get a division by zero and this results in NaNs in the output.

To address this (but not completely fix it, see #89), ODE-toolbox should automatically check whether setting the inhomogenous part to zero results in a division by zero, and if this is the case, issue a warning. This is akin to checking for singularities in the propagators, but it occurs for the update expressions instead.

A flag is added so that singularity detection (also singularity detection in the propagators) can be disabled for the sake of faster processing. On some systems of equations, singularity detection can take a very long time.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.81%. Comparing base (221c577) to head (df13fd0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
+ Coverage   61.71%   61.81%   +0.10%     
==========================================
  Files          15       15              
  Lines        1460     1464       +4     
==========================================
+ Hits          901      905       +4     
  Misses        559      559              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[pnbabu]

Looks good to me, thanks!

[clinssen]

Thank you for the review!