v1.2.0

Latest update: 1.2.0

• Updated to offer both dark mode and light mode for plotting region.
• Removed unused function in Phase Plane app.
• Improved user message when changing the solver field density slider.
• Fixed case choices on slider labels to be consistent with each other.
• Removed ode15i from offered solvers because it is an implicit solver that requires a different equation structure than these apps are designed to study.
• Add CONTRIBUTING.md to describe how to contribute to this repository.
• Update license to BSD-3.