Adding license

[bernardopacini]

Purpose

Niceplots does not currently have a license, meaning it defaults to being strict. After discussing with @eytanadler, I am adding a GPL license here.

Expected time until merged

1 hour.

Type of change

• [ ] 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
• [X] Maintenance update
• [ ] Other (please describe)

Testing

N / A

Checklist

• [ ] I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• [ ] I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• [ ] I have run unit and regression tests which pass locally with my changes
• [ ] I have added new tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation

[A-CGray]

Why is this license file different from the one pyoptsparse uses? That's also GPL