Add us units to MML

[LucileD-met]

Goal

Describe the big picture of your changes here (one sentence). Closes #493 , by adding lbh (pounds per hour), ft3h (squared foot per hour) and inH20 (pressure) to MML flow and pressure sensors.

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 checked that my work is compatible with OpenModelica
• [ ] I have added/updated tests that prove my development works and does not break anything.
• [x] I have made corresponding changes or additions to the documentation (in Notion documentation)
• [x] 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.