Handle input containing division by zero better

[clinssen]

In case of numerical issues with the input, throw a user-friendly error, like:

NumericalIssueException: The variable "zoo" was found. This indicates a numerical problem while solving the system of ODEs. Please check the input for correctness (such as the presence of divisions by zero).

[clinssen]

Sadly, sympy 1.4 seems to not work well with newer Python versions (starting from 3.9). This is probably triggered now by fixing the bug where "Mul" was missing from the list of sympy operators. If #70 is merged, we can drop support for sympy 1.4 altogether. I would suggest to wait for that PR to be merged first.

[codecov]

Codecov Report

Attention: Patch coverage is 94.33962% with 3 lines in your changes missing coverage. Please review.

Project coverage is 61.67%. Comparing base (b692284) to head (beee2a7).

Files with missing lines	Patch %	Lines
odetoolbox/sympy_helpers.py	86.95%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   60.77%   61.67%   +0.90%     
==========================================
  Files          15       15              
  Lines        1420     1456      +36     
==========================================
+ Hits          863      898      +35     
- Misses        557      558       +1     

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[pnbabu]

Looks good to me!