Fix specific heat model with bdf2

[hepap]

Description

The specific heat model in BDF2 was not protected against a division by zero (dH/dT)

Solution

A tolerance is now added to the denominator (dH/(dT+tol)) to avoid this problem

Testing

No change in the tests

Documentation

No change in the documentation

Miscellaneous (will be removed when merged)

I didn't add a new test because I think the current state of the cp model will probably change to be more robust in BDF2

Checklist (will be removed when merged)

See this page for more information about the pull request process.

Code related list:

• All in-code documentation related to this PR is up to date (Doxygen format)
• Copyright headers are present and up to date
• Lethe documentation is up to date
• Fix has unit test(s) (preferred) or application test(s), and restart files are in the generator folder
• The branch is rebased onto master
• Changelog (CHANGELOG.md) is up to date
• Code is indented with indent-all and .prm files (examples and tests) with prm-indent

Pull request related list:

• Labels are applied
• There are at least 2 reviewers (or 1 if small feature) excluding the responsible for the merge
• If this PR closes an issue or is related to a project, it is linked in the "Projects" or "Development" section
• If the fix is temporary, an issue is opened
• The PR description is cleaned and ready for merge

[AmishgaAlphonius]

Nice work! :)