Added pi to constants and Length to Units

[nabily-met]

Goal

Added pi as a constant and Length as a unit. No need to add in changelog.

Resolves #464

Type of change

• [ ] Bugfix
• [x] 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)
• [x] Non-Breaking change

Checklist

• [x] I have added the appropriate tags, reviewers, projects (and detailed the size and priority of my PR) and linked issues to this PR
• [x] I have performed a self-review of my own code
• [x] 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
• [x] 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.

[nabily-met]

Could clean up all unnecessary annotation changes before you merge ? I guess only the Constants package should be modified

These are empty lines that has been automatically deleted by dymola. Do you think I should re-add them? It's only the constants package that has been modified, I also added 2 units, length and mass.