[WIP] Homogeneous reactive physics implementation

[oguevremont]

WIP

Description

There is a growing need to be able to handle reactive systems in Lethe, partly to be able to model chemical reactors. This PR implements a new auxiliary physics able to handle a flexible number of reactive species in homogeneous reactions (for now).

Testing

Documentation

Miscellaneous (will be removed when merged)

Checklist (will be removed when merged)

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Code related list:

• All in-code documentation related to this PR is up to date (Doxygen format)
• Lethe documentation is up to date
• New feature 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