Volume Unit Fix and Numpy Version fix

[Yuksel-Rudy]

Purpose

In the sample.txt file, the volume unit was fixed to (m^3) instead of (mˆ3), this generated a problem in running the example for MacOS users. In this pull request, I also adjusted the unload method in the system.py so that when users create a MoorDyn file, it will have the correct symbol for the volume unit. Additionally, the new numpy version has removed some features that made moorpy obsolete. One easy fix for now is to force the installation of an older numpy version in the environment.yml file. Explain the goal of this pull request. If it addresses an existing issue be sure to link to it. Describe the big picture of your changes here, perhaps using a bullet list if multiple changes are done to accomplish a single goal. If it accomplishes multiple goals, it may be best to create separate PR's for each.

Type of change

• [x] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (non-backwards-compatible fix or feature)
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no API changes)
• [ ] Documentation update
• [ ] Maintenance update
• [ ] Other (please describe)

Testing

Explain the steps needed to test the new code to verify that it does indeed address the issue and produce the expected behavior.

Checklist

Put an x in the boxes that apply.

• [ ] I have run existing tests which pass locally with my changes
• [ ] I have added new tests or examples that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation

[mattEhall]

Has been pulled in from the later pull request.