non zero nominal values

[valentind-met]

Goal

Fix #412 : all nominal values are non zero in base classes (sensors and flow models)

Type of change

• [x] Bugfix
• [ ] New feature
• [ ] Refactoring change
• [ ] Release & Version Update (don't forget to change the version number in package.mo)

Will it break anything in previous models ?

• [ ] Breaking change (If yes, make sure to point it out in the changelog)
• [ ] Non-Breaking change

Checklist

• [ ] I have added the appropriate tags, reviewers, projects (and detailed the size and priority of my PR) and linked issues to this PR
• [ ] I have performed a self-review of my own code
• [ ] I have checked that all existing tests pass.
• [ ] I have added/updated tests that prove my development works and does not break anything.
• [ ] I have made corresponding changes or additions to the documentation (in Notion documentation)
• [ ] I have added corresponding entries to the Changelog
• [ ] I have checked for conflicts with target branch, and merged/rebased in consequence

You can also fill these out after creating the PR, but make sure to check them all before submitting your PR for review.