Fix C formal integral

[chvogl]

Description

This PR makes it possible to run the formal integrator again, which is currently not working.

Motivation and Context

The formal integrator is used in the generation of the documentation and it is a useful feature that users
might want to use. It is important to have a working version.

How Has This Been Tested?

I ran the code and it worked. Hopefully the tests will pass.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have assigned/requested two reviewers for this pull request.

[codecov]

Codecov Report

Merging #1358 (90d3a6a) into master (f07228f) will increase coverage by 0.53%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1358      +/-   ##
==========================================
+ Coverage   71.24%   71.77%   +0.53%     
==========================================
  Files          66       66              
  Lines        5077     5077              
==========================================
+ Hits         3617     3644      +27     
+ Misses       1460     1433      -27     

Impacted Files	Coverage Δ
tardis/tardis/montecarlo/formal_integral.py	25.26% <0.00%> (+9.47%)	⬆️

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